1
GRAIIAMSTOWN SURFACE QUARTZITES.
GRAHAMSTOWN SURFACE QUARTZITES (SILCHETES)
(Read 26th April, 1937.)
By J. J. Frankel 'and Leslie E. Kent.
[PLATES
1-111.]
(Published by permission of the Honourable the Minister of Mines.)
CONTENTS
PAGE
PREAMBLE •.•
THE GRAHAMSTOWN PENEPLAIN,
GEOLOGY AND PETROLOGY
(a)
(b)
(c)
(d)
(e)
I
contributed by
General Introduction
The Underlying Rocks ...
The Silcretes in Hand Specimen
The Silcretes in Thin Section
Heavy Residues ...
CHEMICAL CONSIDERATIONS
OTHER SURFACE DEPOSITS
DISCUSSION OF OBSERVATIONS
CLASSIFICATIONS OF SURFACE DEPOSITS
THE AGE OF THE SILCRETES
ECONOMIC ASPECTS
CONCLUSIONS AND REsUME
ACKNOWLEDGMBNTS
BIBLIOGRAPHY
EXPLANATION OF PLATES
E. D. MOUNTAIN
2
4
4
4
6
8
II
20
27
28
33
34
36
38
40
40
42
PREAMBLE.
Sca,ttered throughout the reports of. the Geological Commission
of the Cape of Good Hope and the geological literature of Southern
Africa in general are references to, and descriptions of, the silicified
rocks formerly known as " Surface Quartzites" but now known as
silcretes in coilformitY"Yith the nomenclature proposed by Lamplugh.
These silcretes are well developed in the Kalahari (23) and the
coastal belt of Southern Africa from beyond the Cunene river on the
west coast, to Kentani near East London, in the east; the most
northerly occurrence reported in the coastal area is at l\fo un (;
Frere (5). Isolated patches are found in the Transvaal and somewhat
larger occurrences are to be ::leen in Rhodesia (12) and the Congo (1&).
The silcretes occurring in the Namib, which are of the Kalah!iri
type, have recently been the subject of a very extensive investigation
by Storz (30). As the coastal occurrences of silcrete differ from those
of the Kalahari, we believe that the results of a detailed investigation of these rocks developed so typicully in the vicinity of Grahamstown, are worthy of record.
Most of the material for this paper was collected whife the
authors were students in the Department of Geology, Rhodes University College, Gr8hamstown. \Vithout thA interest, criticism and
contributions of Proiessor E. D. Mountain, and the facilities to complete the Jaboratory investigations, aff;)rded by the Directors of the
l\1inerals Research Labombry and the Geological Survey, thesA
observations would hnve remained incompIfte.
2
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
t
1Q,,,..
SURFACE QUARTZITE. ON
CRAHAMSTOWN
I
,
"I' ~_1L..
Klngwi
,,wn.
,~"
PENEPLAIN.
"
t'
T.N.
,
SCAl..E. 'N MlL.E.S
\
I
J
"to
"
.,. . . Por~ E.li1.Qbe.th.
FIG.
1.
ContI ibuted by E. D, Mountain,
THE GRAHAl\fSTOvVN PENEPLAIN.
By E. D. MOU!-T'J'AIN, M.A. (Cantab.).
Grahamstown is situated in nn amphitheatre-like depression
which faces south-east and is carved out of a peneplain by the
headwaters of the Kowie River, locally known as the Blaauwkrantz
River. The peneplain was originally continuous from Botha's Ridge
in the north which runs approximately east-west some five miles
north of Grahamstown. to the mountains just south of the city,
while its east-west extent must have been considerably greater. But
it has been severely dissected by the Botha's River, which flows
generally in a north-westerly direction to join the Great Fish
River, by the New Year's River, whkh flows westwards ultimately to
join the Bushman's River, and by the tributaries of the Kowie River,
GRAHAMSTO\YN SURFACE QUARTZITES
3
and now only a tract· of country of highly irregular outline remains
of this peneplain together with a number of small detached flattopped hills. The main area stretches about ten miles east-west
and five miles north-south and covers an area of some, twenty square
miles.
That part of the peneplain just north of the city is known for
obvious reasons as the Racecourse Flats or Aerodrome Flats and
indeed forms a strikingly flat stretch of country. It is traversed in
different places by the main rO'ads out of Grahamstown to Kingwilliamstown, Hunt's Drift, Cradock and PO'rt Elizabeth and only
the road to Port Alfred avoids it altogether. It is, moreover,
extraordinarily horizontal so that, apart from slight depressions
obviously caused by drainage, there is less than thirty feet difference in height between the extreme ends O'f the peneplain in a
N.W.-S.E. direction.
Along this line the edges of the peneplain
very nearly coincide with the 2,100-foot contour-line, but towards
the south-west the level of the surface rises against the mountains
of Witteberg Quartzite and forms a shelf against them at a height
as great as 2,240 feet. At one point on the northern slope of the
Brickfield valley there are twO' conspicuous shelves one above the
other, situated respectively at 2,115 and 2,090 feet, the lower she1£
running about 400 yards while the upper is Gontinuous with the main
flats. Seeing that both are overlying highly weathered Lower Dwyka
Shale, it is possible that the lower shelf has slumped into its position
from above.
Generally speaking, we may say ,that the peneplain coincides
with the distribution of silcrete, the peneplain usually being capped
with a veneer of the latter material up to about thirty feet in thickness. In places, however, outcrops of Witteberg Quartzite are
observed on the surface of the peneplain and hummocks' of this rock,
in places stand O'ut well above the general peneplain level, as is the
case at the Power Station. A conspicuous flat-topped hill just to
the nO'rth of the Port Alfred mad hvo miles from the citv is moreover an outlier of the penepJain, which carries no capping of silcrete.
On the other hand, within the area nnder consideration smal1
platforms of silcrete can be observed at appreciablY,lower leveis than
that of the main peneplain. For instance, a small shelf above the
Old Hospital lies at a little over 1,900 feet, and an iFlolated koppie
capped with silcrete abO'ut three miles from Grahamstown in the
Belmont Valley is elevated at about 1,650 feet, while further down
the valley on Pigot Park is another outcrop at 1,400 feet. It is
interesting to note finally that the deposit at 1,650 feet is si-tuated
at the same height approximately as another deposit on Klipdrift,
along the Port Alfred vVest road and in Howieson's Poort respectively.
The vegeta.tion on the peneplain consists essentially of short
grass invaded or even replaced by shrubby composite weeds of grazing
like the Renosterbush.
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TRANSACTIONS OF TIlE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
GEOLOGY AND PETROLOGY.
(a) GENERAL INTRODUCTION.
The Grahamstown peneplain, in common with all the coastal
peneplains of the Cape Province, was a~tributed by the late Professor Schwarz to processes of marine denudation (27), (28). If
this view be accepted, there would be no inherent objection to postulating that the silcretes lying on the peneplain might be silicified
beach sand.
Rogers and Schwarz, on the other hand, have stated that the
presence of a barrier across the course of a river would result in a
peneplain being cut by the river above that barrier (25). Peneplains
in the folded belt of the Cape Province have been described by
Haughton (10), who regards them as due to the normal 'processes of
attainment of base level of erosion by mature rivers.
1£ the Grahamstown peneplain were produced by such processes
of river erosion, then the silcretes might possibly have formed by, the
silicification of the fluviatile deposits on the peneplain. In other
words, if this were the case the silcretes would be essentially silicified "foreign" material derived from many and possibly distant
sources.
In the vicinity of Bellville Junction, near Capetown, Hatch and
Corstorphine (9) have attributed a small occurrence of silcrete to the
silicification of a sand dune. The possibility that some portion of the
allogenic fraction of the Grahamstown silcretes is aeolian in origin
is thus suggested.
Lastly, there remains the theory that, regardless of the orIgm
of the peneplains themselves, the deposits which subsequently formed
on them and which became silicified are residual and derived from the
weathering of the under lying formations.
'This study of the Grahamstown silcretes was undertaken in the
hope that detailed evidence would be forthcoming which would
enable the nature and origin of the silicified deposits of the Cape
coastal belt generally to be more clearly understood.
The silcretes resting on the Grahamstown peneplain overlie the
Witteberg series, the Lower Dwyka shales and Dwyka tillite, and a
small platform on Klipdrift carries silcrete resting on the Bokkeveld
series.
(b) THE UNDERLYING ROCKS.
Before proceeding to describe the silcretes it is desirable to
mention the characteristics of the underlying formations.
(a)
The Bokkeveld Series. Only the uppermost portion of this
formation, which is folded to a fail1 degree, comes under consideration.
The predominant rock type is shale, but the series is extraordinarily
GRAHAMSTOWN SURFACE QUARTZITES.
5
variable, quartzites and flagstones being interbedded with the shale.
The sand fraction obtained from the shale contains sub-angular and
rounded grains ranging up to 0·3 mm. in diameter, dominantly of
quartz.
(b) The Witteberg Series overlies the Bokkeveld series conformably; a prominent white quartzite which is particularly well exposed
in Featherstone's Kloof being regarded as the base.
The predominant rock type in this series is quartzite, which is associated
with micaceous and sometimes carbonaceous shales.
The Witteberg
serieE, too, is considerably folded.
Numbers of la.rge and small
quartz veins are found in this series.
In thin section typical vVit,teberg quartzite exhibits an equigranular mo.saic texture, being composed of sub-angular and angular
quartz grains only occasionally showing any degree of rounding, with
a small anwunt of interstitial flakes and shreds of sericite, often ironstained. Occasionally a slight secondary enlargement of a small percentage o.f the quartz grains was no.ted; this seems to be a feature
of the Witteberg, particularly neRr its junction with the overlying
silcretes. Fretting and embayment of the quartz grains, however,
is far more noticeable as a rule.
The average grain size of the allogenic: quartz is approximately
0·2 mm., the grade limits being between 0·01 and 0·7 mm. Only
relatively few of these grains show undulose extinction and some
exhibit "bohmische streifungen," indicating that the vVitteberg is
partly derived from metamorphosed formations.
No felspar grains were noted in the thin sections examined.
Composite grains of quartzite are, ho.wever, fairly common. Accessory
minerals noted were dark; yellow well-rounded rutiles up to. 0·16 x
0·6 mm. in size, small rounded zircons and minute grains of dark
iron ores. l\1easurements made by means of the Shand recording
micro.meter gave the average percentage o.f allogenic grains to
seco.ndary sericite as 94 to 6.
(c) The Lower Dwyka Shale. Following conformably on the
Witteberg series is this formation composed o.f shales, with a variety
of sandstones and fiagstoHes.
The shales have suffered intense
weathering, and an average of at least 50 feet of clay has been produced. In this clay there is an abundance of quartz crystals, many
of which are doubly terminated and appear to be prevalent in certain
zones which are more evident nearer the top of the clay bank.
The unweathered rock is so.metimes slaty, pyritic and o.ften
carbonaceous. The rock in thin sectio.n is a typical shale, being
composed essentially of an argillaceous ma.trix with disseminated
quartz grains, sericite and limonitic material.
The sand fraction from the shale consists of sub-angular and
rounded quartz grains, being on the average between 0·01 to 0·5 mm.
in diameter.
.
()
TRANSACTIONS OF TIlE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
(d) The Dwyka Tinite. This formation, too, is highly weathered
to a depth of at least 30 feet, and has formed clays which differ !r?m
those derived from the shales in being more sandy and contammg
minute angular and sub-angular fragments of quartz. This difference
enables the boundary between Lower Dwyka shales and Dwyka tillite
to be mapped with a fair amount of accuracy. rrhere is also a large
number of quartz veins in the tillite.
In thin section Dwvka tillite is seen to consist of angular pebbles
and grains, dominantly" of quartz, set in a fine argillaceous matrix.
On viewing the matrix with high magnification it is often seen to
have a banded structure due to the presence of parallel layers anp.
lenticles alternately light and dark in colour, which make the rock
appear bedded. These dark bands, which are 20 microns in thickness
on the average, twist and curl round the larger grains.
The allogenic portion consists of small angular pebbles of finegrained sericitic quartzite, coarse and fine-grained granular quartzites,
weathered granite, pegmatite rnaterial, chert, felspar, etc., together
with quartz grains which varY' in size from the silt grade to large
grains about one cm. in diameter. These quartz grains, of which a
large percentage show undulose extinction and "bohmische streifungen, " are usually not emhayed and fretted and are highly angular
in outline, occasionally long thin laths being noted. Felspar, usually
microcline, but quite commonly plagioclase, is an important
constituent.
Accessory minerals noted were zircon, altered augite, large pink
garnets up to 0·4 x 0·35 mm., muscovite, chlorite, altered common
hornblende, minute allogenic rutiles, bleached biotite, green epidote,
tremolite, specks of iron ores, etc. The harder minerals in this rich
assemblage of accessories are usually well rounded as compared with
the quartz grains, doubtless due to their derivation from pre-existing
sediments.
The groundmass 1S composed of fine.-grained argillaceous
material which contains abundant flakes of sericite.
rrhe average
relative volume percentages of the components were determined with
the recording micrometer ·as follows, pe:bbles 'being neglected:Quartz: Felspar : Accessories: Ground mass: : 27 : 9: 2: 62.
It might be expected that because of the difference in characteristics of the underlying rocks mentioned above a certain amount of
variation in the nature of the surface deposits would be noticed.
Aceordingly the silcretes and the immediately underlying rocks were
examined macroscopically, in thin section and chemically, and heavy
residues were separated and investigated.
U
(c) THE SILCRETES IN HAND SPECIMEN.
The silcretes show smooth hummocky surfaces which on weathering, take a good polish as a rule, but irregular pocked and pitted
weathered surfaces are found, sometimes within a few feet of the
GRAHAMSTOWN SURFACE QUARTZITES.
7
former. The colou!: of the fresh rock varies from grey to whitish and
buff. Usually the weathered surfaces are a yellowish to light brown
colour. Whatever the type of silcrete, all appear to have an external
concentration of iron which imparts the characteristic colour.
The thickness of the formation, even though it transgresses across
formations of vastly different sand content, is fairly uniform, approximately 20 feet thick on the RV8ragc, although a thickness of 35 feet
is found to the south of the brickyards valley. In general, the silcrete
on the Witteberg is the thickest. Present and past erosion has
probably reduced the original thickness to some extent. The greater
part of the silcrete occurr;~nces on the peneplain, however, overlie the
Dwyka tillite.
In a hand specimen, the silcrete is seen to be composed of small
angular to sub-angular quartz grains held in a ha,rd siliceous matrix;
sometimes the rock is extremely fine-grained with scattered quartz
fragments. Coarser types are also met with, the fragments of quartz
being over half an inch long. A type is also found in whlCh pebbles
of silcrete are found in a matrix of younger silicified material. The
silcretes are compact and unbedded; sometimes, however, the concentration of iron takes place along rough planes and a slightly bedded
appearance is assumed. The rock invariably breaks across grains with
a conchoidal or sub-conchnidal fracture. Jointing is sometimes seen
on a l~rge scale, being well exhibited at Sugar Loaf Hill.
A peculiar feature of all the silcretes examined, including those
from other areus, is the pl'csence of cavities. These cavities are
usually small, occasionally up to half an inch in length, and are
elongate. "They sometimes occur in roughly horizontal la,yers, and
there appears to be a general tendency for them to occur in strings,
or rather, in planes, associated with a very fine-grained "pasty"
material. There is a possibility that they may be associated with
small-scale jointing.' '1 Sometimes the cavities contain small terminated crystals of quartz.
Although not common on the Gmhamstown peneplain, several
exposures have revealed evidence for different periods of silicification~
At t,he Settlers Hospital, for example, the silcrete resting immediately
on the Dwyka tillite is what might be termed a ., silcrete conglomerate, ., for it contains silcrete pebbles. rIhis grades into a
brecciated silcrete set in a ferruginous matrix, and thut into a typical
medium-grained silcrete. To the south of the Port Elizabeth road,
opposite the New Cemetery, the base of the surface deposits is a
" silcrete conglomerate " which is aRsociated vvith and grades into
brecciated silcrete having a ferruginous matrix.
" Silcret,e conglomerate" is also fairly abundant in the deposits
to -the west of. Mossel Bay, where the base of the rock overlying
Bokkeveld slates is often of this type. In the outliers of silcrete
occurring in the Gamtoos Vallpy (7), "silcrete conglomerate" has
1
Personal communication' from Professor E. D. Mountaiu.
8
TRANSACTIONS OF THE GEOLOGICAL SOCIE'fY OF SOUTH AFRICA.
been found at the base resting on pre-Ca.pe phyllites. This conglomerate is associated with brown -ferruginous cemented sands, and
the two varieties, which appear to grade laterally into each other,
are overlain by massive medium-grained silcrete.
Around Grahamstown no brecciated Witteberg quartzite has,
however, been seen overlain by massive silcrete. In Featherstone's
Kloof a small patch of silcrete is composed of slightly rounded
Witteberg pebbles set in a normal silcrete matrix. Haughton, however, has recorded from Klipdrift (Albany Division) that: "Along
the eastern boundary of this farm northerly dipping Witteberg quartzites have their upper surfa.ce brecciated for a foot in depth, and
the fragments are cemented by a ferruginous eement. This represents the extreme base of the continental plulse in this area" (11c).
No quartz veins have been found in t,he silcretes.
(d) rfHE SILCRETES IN THIN SECTION.
In hand specimen it is often possible to distinguish the more sandy
uniform silcrete overlying the \Vitteberg from those occurring on the
Lower Dwyka shale and Dwyka tillite which are, especiaUy the latter,
more variable in grain size. The Dwyka tillite silcrete moreover generally contaIns pebbles of vein quartz to a far greater degree than the
others.
To demonstrate the val'lation in silcrete overlying different formations, measurements were made on several sections, and there is
a definite difference in the percentage ratios of allogenic grains,
which we take here as grains gl'ea,ter than one micron, to the secondary quartz and adsorbed clay fraction. rfhese percentage ratios are
tabulated at the end of this section. tfhe camera lucida drawings
show rather clearly the difference of allogenic to authigenic fractions
in the silcretes from the different underlying rocks.
Silcrete overlying Bokkeveld Beds. 'fhis is formed dominantly of an
assortment of slightly rounded, sub-angular and angular qua,rtz grains,
of which the sub-angular are by far the most abundant, held in a
chalcedonic matrix. The diameter Of the majority of the grains varies
from 0-01 to 0-5 mm., the a,verage being about 0·5 mm. A small percentage of these grains exhibit slightly fretted outlines and embayments, and a few show an undulose extinction.
A few rounded fragments of \vhat appears to be Bokkeveld
quartzite occur sparingly throughout the silcrete-secondary silica
has often permeated these between grains. Older fairly well rounded
silcrete grains and pebbles varying from 0·2 x 0·1 mm. to large
pebbles have been noted; these can as a rule be clearly differentiated
from the younger silcretes by the deeper staining of the matrix of
the older SIlcretes.
Np felspar grains were noted in the slides examined. A few
willI rounded zircons, rutile and iron ores are scattered throughout
the~ matrix.
Under high magnifieation a few thin rutile needle~
GRAHAMSTOWN SURFACE QUARTZITES.
9
can be seen sporadically d.istributed throughout the grou(lldmass.
Cubes of limonite having an average size of 0·06 mm. can also be
seen occasionally.
Colloform whorls are present in the crypto-crystalline groundmass and appear to be' confined to those spaces in which quartz fragments are not abundant. These were only noted microscopically, the
average size of the structures being about 0·15 mm.
Silcrete Overlying the Witteberg Series. The silcrete overlying
Witteberg quartzite is characterised by its relatively equi-granular
nature as compared with the silcrete found on the other formations,
even though slight variations in the degree of " packing" of grains
in the rocks from different localities were noted. The grains have an
average diameter between 0·08 and 0·5 mm.; larger individual grains
being about 0·8 mm. :Most of the grains are irregular, sub-angular
and angular, only a very small number being at all well rounded.
The majority of these quartz grains are fretted and rather ragged
at their edges, and are I)ften markedly embayed; this is generally
the case, but in a few of the slides examined only traces of fretting
could be detected.
None of the grains seen showed any signs of secondary enlargement. Few show effects of strain. No felspars or even replaced
Pebbles of silcrete are of quite frequent
felspars were observed.
occurrence and are genera]]y differentiated from the cementing
silcrete by differences in the respective degrees of iron staining.
Their margins with the younger silcrete are often vague and they
often seem to grade into each other, as the secondary silica permeated
the matrix of the older silcrete.
In the slides examined the older" silcrete matrix, which is
generally much less iron stained, seems to have undergone more
profound alteration than the younger, the secondary silica. being
(jfteh observed to have recrystullised to micro-crysta,lline quartz.
Pebbles of \Vitteberg quartzite, irregular to slightly round in outline,
are of frequent occurrence. rrhese have often been permeated by the
secondary silica along grain margins.
No sericite, which is so
common in the Witteberg itself, was observed in the overlying silcrete
in which it appears to have been repla.ced by silica.
The degree to which the pebbles have been replaced and permea.ted by authigenic silica is obviously a function of the extent to
which they weathered. In the larger cracks in these pebbles normal
silcrete matrix is visible.
Minute and medium-sized zircons, detrital rutiles, and a few
common hornblendes were noted. Specks of iron ores are common
in the matrix, and these appear to be especially concentrated along
the margins of the larger grains. SOlle of these iron oxides appear
to have recrystallised. The matrix is composed of very fine-grained
detrital material of the silt grade set in authigenic chalcedony. The
matrix is usually semi-opaque due to later iron staining.
10
TRANSACTIONS OE' THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Distinct colloform structures in the chalcedony were noted
both in the silcrete pebbles and in the more recent ~ilcrete matrix.
They are confined to the spaces between large grains and are consequently usually very small in size-the largest noted being 0·3 x
0·2 mm. In sectIOns of the Featherstone's Rloof silcrete, irregular
bands, patches and streaks, usually about one mm. across, of finegrained material occur in the normal silcrete. Under high magnifi:cation the fine-grained mat,edal contains much more fine-grained
allogenic material in the interstices between the larger grains than
the normal silcrete. They may possibly represent lenses of sandy
clay.
Silcrete on Lower Dwyka Shales. rr1;lis consists of angular quartz,
grains set in a matrix of Vf~ry fine, angular quartz grains cement~d
by crypto-crystalline chalcedony. Few grains are sub-angular, nonA
are well rounded, and a small number only show secondary enlargement. A large number of the grains are irregulllr and e:p::tbayed, and
a few show solution effects.
Undulose extinction and other strain phenomtma are exhibited
in a fair proportion of the quartz grains. The majority of the grains
are 0·1 mm. in diameter, with htrger .grains. up to 0·5 mm., but on
the whole a greater percentage of minute grams is to be seen in this
rock than in. the other silcretes.
A few sub-angular zirc-.ons, paJe yellow minute rutile needles
and iron ores were noted.
Felspar ·iSi, however, entirely absent.
Very vague colloform structures are occasionally present in the larger
interspaces. The groundmass of.crypto7crystalline silica contains ~
small amount of adsorbed day materia:! and iron oxides.
Silcrete on Dwyka Tillite. Set in a matrix of crypto-crystalline
silica are quartz grains mostly angular and occasipnally sl,lb-angular:~
A characteristic of this silcrete is the variability in gram size-a
striking difference when cc.mpared with the uniform size of grllin III
silcrete overlying the vVittebel'g quartzites. The grains vary in siz.e
from minute fragments to larger grains 0·5 mm. in diameter; a large
percentage of the grains have nn average diameter of 0·2 mm. Few
of the quartz grains are embaved or fretted; apparent embaymentEi.
however, are merely edges of grtljns irregularly covered with m.atrix
ironstruined as a rulE'.
Quite a number of grains have been
partly and completely replaced by second::try silica. A careful search
in all the slides examined failed to reveal any secondary enlargement
Strain phenomena are present in a large proportion of the Quartz
grains.
Felspar, from fairly fresh to highly weathered fragments, occurs
in small quantities, rarely constituting even one per cent. of t.he
rock. Another indication of the variable nature of this silcrete which
may be mentioned here ~s the fact that in some of the slides
examined no felspar was noted at all.
Orthoclase and microcline.
11
GRAHAMSTOWN SURFACE QUARTZITES.
together wjth somewhat less plagioclase, were recognised, and quite
often these minerals hl;we been repla.ced by silica.
Pebbles of sheared quartzite and vein quartz up to 0·3 mm.
III diameter as well as a few of resilicified weathered granite are
scattered throughout the silcrete. t;ome of the quartzite pebbles are
composed of interdigita,t,ing and embayed grains, which fact no doubt
llccounts for some of the embayed grains in the silcrete. In addition
to these pebbles of pre-silcrete age '!jhere are also a few pebbles of a
finer grained compact silcrete whose margins with the younger silcrete
matrix are often rather vague. The quartz grains in these silcrete
pebbles are angular and some are slightly rounded.
Some of the very fine-grained silcrete pebbles may originally
have been sandy pebbles in the Dwyka tillite, but due to subsequent
silicification they now appear as typical fine-grained silcrete. Finegramed bands of iron-stained material up to 3 mm. in thickness often
occur in the rock, and appear to be distributed in rude bands or
zones, which contain a few large quartz grains. The bands have a
" pasty" appearance and may have been lenses of silt or clayey
material which after silicification have cracked and produced voids.
The heavy minerals noted i'n the sections were yellow-orange
rutiles, a few zircons and a small quantity of iron ores. Occasionally
a few needles of secondary rutile were observed. The matrix of the
silcrete is often iron-stained and semi-opaque, and although it is
extremely fine-grained high magnifications reveal it a.s chalcedony.
Perfect colloform whorls of alternating light and dark bands are
present in the matrix, and they are occasionally Iound round and
along the sides of some of the quartz grains.
'rhe differences in the ratios of authigenic to allogenic constituents of the silcretes overlying the four formations which occur
round Grahamstown are clearly shown in the following table:Allogenic.
Silcrete overlying.
56
90
52
45
Bokkeveld
Witteberg
Lower Dwyka Shales ...
Dwyka Tillite
(e)
HEAVY
per
per
per
per
cent.
cent.
cent.
cent.
Authigenic.
44 per cent.
10 per cent.
48 per cent.
55 per cent.
(This last figure is
approximate as the
Dwyka tillite silcrete is extremely
variable.)
RESIDUES.
Heavy residues were obtained by bromoform separation.
Preliminary check experiments showed that crushing of the silcrete
to at least 200 mesh was necessary to obtain good yields of heavy
minerals. This mesh was the smallest available, but doubtless jf
12
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
still smaller mesh sieves had been used, even larger residues containing, for example, the minute needles of rutile noted in the thin
sctions, would have been obtained.
A rough experiment on mat,erial very finely ground in the agate
mortar indicated that some of the rutile needles could be recovered
by centrifuging the powder jn bromoform.
In order to bring out more clearly the differences evident from
this work, a comparative table showing the characteristics of the
heavy residues has been dra,wn up. '
The precentages by weight of the different component minerals in
the residues, including quartz having a high specific gravity due to
iron ore inclusions (later subtracted from the total), were determined
by counting all the grains in the smaller residues and many typical
fields in the larger. Allowances were made for flat grains in the
ca.lculation of weight percentages from the area measurements.
When only a few grains were noted in the residue the result is shown
merely as a trace (tr.).
Table of Shape Nomenclature.
=
I Irregular.
A=Angular.
SA = Sub-angular.
SR = Slightly rounded.
FWR = Fairly well rounded.
WR= Well rounded.
and D == Average mean diameter of undoubted detrital
grains (all grains obviously fractured in the mortar
neglected).
TABLE I (L.E.K.)
Locality: Klipdrift at the base of W oest Hill.
UPPER BOKKEVELD SHALE.
SILCRETE OVERLYING.
(Not sampled).
Weight taken, 200 grams.
Weight taken, 270 grams.
Crushed to 200 mesh.
Crushed to 200 mesh.
Weight per cent. of Heavy Minerals: 0·004.
Weight per cent. of Heavy Minerals: 0·60.
Details of Heavy Minerals
and per cent. by weight.
D
Shapes.
Remarks.
-
-
-
-
ILMENITE
...
...
15
0·07 mm.
Majority SA
to SR
Few A and 1.
LEUCOXENE
...
15
0·07 mm.
Majority A
some FWR.
TOURMALINE
...
(mostly Dravite)
tr
MAGNETITE
-
-
Details of Heavy Minerals
and per cent. by weight.
Shapes.
Remarks.
MAGNETITE
...
tr
0·05 mm.
SA to WR.
ILMENITE
...
16
0·06 mm.
Mostly SA to
FWR.
FewA.
LEUCOXENE
...
1
0·07 mm.
Only a few small brown
crystals noted.
TOURMALINE ...
(mostly Dravite)
3
0·12 mm.
A to SR.
Brown and olive-brown
in
colour.
Large,
hardly
any
smaller
than 0·1 mm.
Mostly altered on surface to Leucoxene
I
D
-
-
Mostly corroded.
Mostly altered on surface to Leucoxene.
-
RUTILE
.,.
...
30
0·08 mm.
SA.
Some very elongate.
Max. noted
(0·16xO·07 mm.)
Ratio of yellow to deep
orange-red 4/3.
Red rutile dark.
RUTILE
...
....
30
0·07 mm.
Mostly A to
SA, acicular
form common,
Mostly very elongate.
Max. noted
(0·24x 0·04 mm.)
Yellow: red= 11 : 9
ZIRCON
...
...
40
0·07 mm.
Mostly WR,
only a few
slightly A at
ends.
All very much the same
size.
ZIRCON
...
...
50
0·08 mm.
Mostly WR,
only few A at
terminations
but at least
half elongate.
On whole large and
fairly uniform in size.
COMMON GREEN
HORNBLENDE ...
tr
0·1 mID.
SA,
...
tr
-
COMMON GREEN
HORNBLENDE ...
-
-
-
-
...
-
-
-
-
LIMONITE
LIMONITE
I
N.B.-No Bokkeveld sandstone samples were available.
-
A few corroded cubes
after pyrites.
TABLE II (L.E.K.)
Locality: Begelly (Brakfontein) 6 miles South of Grahamstown near base of the Witteberg Series.
Locality shown on Cape Sheet No.9 (Port Elizabeth) of the Geological Survey.
WITTEBERG QUARTZITE
SILCRETE OVERLYING
I
Weight taken: 100 grams.
--_.
Weight taken: 100 grams.
Crushed to 100 mesh.
Crushed to 200 mesh.
Weight per cent. of Heavy Minerals: 0·045.
Weight per cent. of Heavy Minerals: 0·103.
Details of Heavy Minerals
and per cent. by weight.
D.
Shapes.
Remarks.
emIrregular,
few
bayed: surface alteration
leucoxene
to
common.
Details of Heavy Minerals
and per cent. by weight.
Remarks.
Many embayed. Surface
alteration to leucoxene
common.
...
13
0·06 mm.
A to WR,
mostly SA.
LEUCOXENE
...
1
0·06 mm.
SA to WR,
mostly FWR.
>1
0·10 mm.
SR to WR.
Few true brown, most
are brownish-green.
...
16
0·06 mm.
SA to FWR.
LEUCOXENE
...
34
0·06 mm.
SA to WR.
<2
0·09 mm.
SA,
Mostly
few FWR.
Dominantly brownishbrown
green. True
and rarely blue noted.
TOURMALINE
(Schorlite-Dravite)
TOURMALINE
(Schorlite-Dravite)
Shapes.
ILMENITE
ILMENITE
-
D.
-
RUTILE
...
...
25
0·07 mm.
Dominantly
Some
SA.
FWR.
Uniform size, slightly
elongate.
Few deep
red. Dominantly yellow and orange-red.
O-red: y .. 3 : 4.
RUTILE
...
...
8
0·07 mm.
Dominantly
SA few FWR.
Few deep .red. Fairly
uniform size.
Most
slightly
e Ion gat e.
O-red: Y:: 7: 8.
ZIRCON
...
...
23
0·07 mm.
All somewhat
rounded.
Only few elongate, uniform size. Max. noted
was (0·25X 0·08 mm.)
ZIRCON
...
...
77
0·06 mm.
Most
WR.
None A.
Few elongate. Most of
uniform size
(0·16x 0·05 mm.) Max.
noted.
EPIDOTE
...
...
tr
0·03 mm.
I and SA.
-
EPIDOTE ...
...
minute
trace
-
-
-
...
-
-
DIOPSIDE
...
minute
trace
-
-
-
-
-
-
DIOPSIDE
BIOTITE
CHLORITE
...
...
...
-
minute 0·1
trace
tr
0·1
-
...
mm.
-
Frayed altered flakes.
BIOTITE
mm.
-
Pale green flakes may
be altered biotite.
CHLORITE
...
tr
0·1
mm.
-
Pale green with magne.
tite inclusions appear to
be altered biotite.
TABLE III (L.E.K.)
Locality: Small peneplain in Featherstone's Kloof, 1,650 ft. (one-third way up
Witteberg Series).
SILCRETE
Weight taken: 120 grams.
Main Grahamstown Peneplain very near top of Witteberg Series.
Locality:
SILCRETE
Weight taken: 150 grams.
Crushed to 200 mesh.
Crushed to 200 mesh.
Weight per cent. of Heavy Minerals: 0·042.
Weight per cent. of Heavy Minerals: 0·015.
Details of Heavy Minerals
and per cent. by weight.
D.
Shapes.
Remarks.
Vast majority have
undergone
surface
alteration to leucoxene.
ILMENITE
...
13
0·07 mm.
Dominantly
SA to WR.
Few A and I
embayments
fairly common.
LEUCOXENE
...
I
0·07 mm.
SA to WR.
tr
0·08 mm.
FWR.
Greenish-brown.
TOURMALINE
(Schorlite-Dravite)
-
Details of Heavy Minerals
and per cent. by weight.
D.
Remarks.
Shapes.
ILMENITE
...
33
0·07 mm.
I and SA to
WR.
LEUCOXENE
...
2
0·07 mm.
SA to WR.
t
0·08 mm.
SA to SR.
Brownandgreenish-blue.
TOURMALINE
(Schorlite-Dravite)
Surface alteration
leucoxene common.
to
-
RUTILE
...
...
24
0·08 mm.
Vast majority
SA, few FWR.
Fair number elongate
more than in other silcretes on Wb, but not
nearly to the same
extent
in Bv
as
R: Y=IO: 9 few yellow, large
number
crimson.
RUTILE
...
.. .
9
0·07 mm.
SA to WR.
majoritysho~
signs of rounding. None acicular.
Fair number crimson,
small number elongate.
but mostly fairly uniform
in size.
Orange-red:
yellow = 3 : 2.
ZIRCON
'"
...
62
0·07 mm.
SA to WR,
mostly FWR.
Size fairly constant,
but large number elongate (0·2XO·04 mm.)
Max. noted.
ZIRCON
...
...
56
0·07 mm.
Mostly SA to
WR, few
highly A.
Few elongate, but
majority uniform in size.
GARNET
(Almandite) '"
CHLORITE
...
...
minute 0·1
tr
-
mm.
-
-
-
GARNET
(Almandite)
-
-
CHLORITE
...
...
minute 0·1
tr
tr
mm.
-
-
Pale pink.
-
A general residue from the Witteberg Series was obtained by panning the sand from a stream in Howieson's Poort, whose entire catchment area is composed
of Witteberg quartzite. The minerals noted in order of abundance are: Ilmenite, zircon, rutile, magnetite, leucoxene, tourmaline (dravite-schorlite), common
hornblende, epidote, pink almandite garnet and augite; the last four being present as traces only.
TABLE IV (L.E.K.)
Locality: Head of Blaauwkrantz River Valley.
LOWER DWYKA SHALE.
GENERAL SAMPLE OF TOPMOST SHALE.
SILCRETE OVERLYING.
Weight taken: 200 grams.
Weight taken: 200 grams.
Washed out clay.
Crushed to 200 mesh.
Left 16 gms. sand, which was bromo formed directly.
Weight per cent. of Heavy Minerals: 0·009.
Details of Heavy Minerals
and per cent. by weight.
D.
Weight per cent. of Heavy Minerals: 0·043.
Shapes.
Remarks.
Details of Heavy Minerals
and per cent. by weight.
-
ILMENITE
...
4
0·06 mm.
Mostly
SA,
few A and R
-
LEUCOXENE
...
18
0·07 mm.
SA to FWR.
-
0·2
0·1
A to SA, few
SR.
Shades of green to
brown.
tr
ILMENITE
...
6
0·08 mm.
SA to SR.
LEUCOXENE
...
39
0·07 mm.
SA to WR,
mostly FWR.
0·5
0·1
SA to FWH.
Olive brown colour.
TOURMALINE
(Dravite-Schorlite)
TOURMALINE
(Dravite·Schorlite)
mm.
Remarks.
...
...
-
Shapes.
MAGNETITE
MAGNETITE
-
D.
Mostly embayed and
fretted.
-
-
-
-
mm.
Many fretted.
RUTILE
'"
...
17
0·08 mm.
Dominantly
SA to SR.
Orange coloured dominant among " red."
Red to yellow 1 : 1.
RUTILE
...
...
39
0·08 mm.
Mostly 1. A
to SA to SR.
Some elongate. Red to
yellow 1: 1.
"Red"
dominantly orange-red.
ZIRCON
'"
...
38
0·06 to
0·07 mm.
Mostly FWR.
Large ones elongate.
Hardly any angular at
ends.
ZIRCON
...
...
40
0·06 mm.
Mostly FWR
toWR.
More
usual.
COMMON GREEN
HORNBLENDE ...
tr
0·15 mm.
SA and 1.
LIMONITE
...
CHLORITE
...
GARNET
(Almandite) '"
...
-
COMMON GREEN
HORNBLENDE ...
tr
CHLORITE
tr
0·1
GARNET
(Almandite)
...
tr
0·12 mm.
-
-
-
LIMONITE
-
-
-
-
-
-
-
~
...
...
-
-
-
mm.
elongate
-
than
-
Trace of cubes.
Cubes after pyrites.
I flakes.
Green colour.
SA and 1.
Pale pink.
TABLE V (L.E.K.)
Locality: Cradock Road.
WEATHERED TILLITE IMMEDIATELY BELOW SILCRETE.
BASE OF SILCRETE OVERLYING.
Weight taken: llO grams.
Weight taken: 120 grams.
Crushed to 100 mesh (sand fraction: 42 per cent).
Crushed to 200 mesh.
Weight per cent. of Heavy Minerals: 0·0036.
Weight per cent. of Heavy Minerals: 0·014;
Details of Heavy Minerals
and per cent. by weight.
ILMENITE
...
LEUCOXENE
...
3
D.
0·08 mm.
Shapes.
SA to SR.
I
TOURMALINE
(Dravite-Schorlite)
I
24
mm. -SAto FWR.
0·1
Remarks.
Mostly altered on surface to Leucoxene.
-
Details of Heavy Minerals
and per cent. by weight.
Shapes.
D.
Remarks.
ILMENITE
.. ,
25
0·08 mm.
SA to FWR.
-
LEUCOXENE
...
6
0·07 mm.
FWR.
-
<1
0·08 mm.
SA.
Shades
brown.
1
0·12 mm.
A to SA.
Shades between green
and brown.
TOURMALINE
(Dravite-Schorlite)
of
green
to
RUTILE
...
...
2
0·07 mm.
A and SA to
SR,
mostly
elongate.
Red to yellow 4 to 1.
mostly
semiRed
opaque.
RUTILE
...
.. .
23
0·07 mm.
Mostly
A,
also SA to R.
Red to yellow, 4 to 7.
Red dark. Some lemon
coloured.
ZIRCON
...
...
35
0·08 mm.
SA to
few A.
Compared with other
formations, zircon large
and variable in size up
to (0·3xO·06 mm.).
ZIRCON
...
...
45
0·07 mm.
SA to
few A.
Variable in size, often
elongate.
BIOTITE
...
...
tr
-
Practically completely
chloritised flakes.
BIOTITE
...
.. .
tr
-
-
CHLORITE
...
tr
about
0·15 mrn.
-
Greenish colour.
CHLORITE
...
-
-
-
-
GARNET
(Almandite) ...
...
35
0·15 mm.
-
-
-
-
-
WR,
A to SA, some
SR.
Pale pink.
IGARNET
.. (Almandite)
.. .
WR,
Partially
flakes.
chloritised
I
NOTE.-A general sample of silcrete from the top was also investigated, the percentage residue in it being 0·008. There was no ~ssential difference between its
composition and the one from the base recorded above, save that it contains about 10 per cent. by weight of magnetite. Further, a specimen of tillite
from the railway cutting on the Cradock Road was found to contain 71 per cent. by weight of sand and 0·14 per cent. of heavy minerals, dominantly
almandite garnet.
18
TRANSACTIONS OF THE GEOLOGICAL SOCIE'L'Y OF SOUTH AFRICA.
TABLE VI (L.E.K.)
Locality: Fort Grey, near East London.
SILCRETE OVERLYING KARROO DOLERITE
Weight taken
100 gms.
Crushed to '"
200 mesh
Weight per cent. of Heavy Minerals
0·07
DetBils of Heavy Minerals
and per cent. by weight.
MAGNETITE
r
D
-
tr
... I
ILMENITE
LEUCOXENE
RUTILE
...
ZIRCON
...
-
Rems,rks.
-
92
0·06 mm,
SA and I,
spme SR
4
0·1
mm.
SA and I
to SR
1
0·1
mm .
Ato SR
Mostly
elongate
(0'14xO'06) red to
orange-redincolour.
2
0·1
mm .
Mostly SA and A,
fewWR
Some show signs of
rounding,
others
have sharp terminations.
COMMON
HORNBLENDE
tr
-
BIOTITE
...
tr
-
CHLORITE
...
tr
0·1
1
0·06 mm.
,
II
-
I
GARNET
(Almandite)
Shapes.
-
mm .
Mostly altered at
surface to Leucoxene and limonite
coated.
-
-
Ragged flakes
(0·1 X 0·06 mm.)
Altering to chlorite.
Irregular flakes
Pale green.
A
-
I
In the light fraction aU minute grains show aggregate polarisation and the refractive index of the matrix agrees with that of
chalcedony. No opal was noted.
Unfortunately no specimens of the underlying Karroo dolerite
were available in the Rhodes University College collection, but
residues from weathered dolerites in Natal (L.E.K.) have, in addItion
to' the dominant magnetite and ilmenite, been found to contain
appreciable quantities of zircons, and less frequently rutile also.
rrhese latter usually showed signs of rounding, possibly caused by
chemical Hction during weathering. Garnets were, however, not
recorded from the dolerites examined.
It is apparent from R study of the heavy residue tables that in
general close relationships exist bet,ween the heavy mineral suites
in the silcretes Rnd those in the immediately underlying rocks. An
exception to this is the case of the Dwyka tillite and the silcrete on
GRAHAMSTOWN SURFACE QUARTZITES.
19
it. However, further residues examined have served to explain this
apparent discrepancy.
A general sample of the soil derived from the weathering of
silcrete overlying the tillite was separated, and the residue was found
to eonsist dominantly of magnetite (50 per cent. of residue obtained
from soil by weight), ilmenit.E\ zircon, rutile, leucoxene with subordinate amounts of almandite garnet and tourmaline. \Vhen the
individual minerals in the soil eoncentrate were further examined
even greater differences were found with the residue from silcrete
tabulated in Table V. For example r the rutiles in the soil were for
the most part of a vivid scarlet colour. An interesting fact disclosed
by the weathered-out residue was tha,t the magnetites were of an
average size of 0·5 mm., which is much greater than those obtained
from a specimen of weathered tillite which was disintegrated under
water and panned.
Field studies have, in addition, shown that the heavy mineral
suite in the tillite 'is verY' variable, as indeed would be exp~cted in a
rock of this type. frhus the apparent discrepancy in the residues
from the tillite examined in detail proves on consideration to be
valuable conurmation of the fact that the silcretes contain heavy
residues similar to those in the underlying rocks. No such variation
was noted ever in the case of the other formations examined, even in
the Witteberg, where three residues from widely separated silcretes
were investigated.
It will have been observed that fundamentally the Bokkeveld,
Witteberg and Lower Dwyka shales contain similar residues; but the
habits of the component minerals are usually vastly different in the
different formations. For example, no other residue exhibited such
a large proportion of highly elongated and acicular rutiles as did the
Bokkeveld and the silcrete on it; and many similar cases will be
apparent from t.he tables.
A significant fact is th~t the shapes of the component minerals
do not alter from underlying formations to silcrete-this surely indicates no transport.
Further, the heavy residues are uniformly greater to a, considerable degree t,han those in the underlying formations. This fact is
proof of th8 theory that the silcretes are silicified residual deposits,
and as such the heavy stable mineral content gradually increased in
the material left in situ after weathering and chemical action had
.removed the easily dissolved minerals.
The increase in residue is not necessarily accompanied by an
increase in each component mineral, leucoxene, for example, being
invariably very much less in the silcrete than in the underlying rock.
Valuable confirmation of the thesis that the heavy residues in silcrete
from this region generally corresponds very closely with tha,t from the
underlying rocks is provided by Table VI, where the residue from a
silcrete on dolerite corresponds closely with those known to occur in
Karroo dolerites. (L.E.K.)
20
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
CHEMICAL CONSIDERATIONS.
On the attainment of perfect peneplanation, the only factor of
importance in weathering is chemical action (32) ; mechanical erosion,
save only possible aeolian deflation, is, almost completely non-existent.
The conditions prevailing around Grahamstown prior to the formation
of the surface deposits must have been such that after the attainment
of the almost perfect peneplain, chemical action was predominant.
To obtain some idea of the part played by chemical action in the
formation of the -surface deposits, a certain amount of analytical work
was done. The Lower Dwyka shales underlying the silcrete have
been weathered to a clay which is used in the Ceramic industry. An
ascending order of analyses from shale low down in the clay pits to
clay just below the silcrete has been selected from previously
published work (16). The analyses are of material from the brickworks near the head of the valley of the Blaauwkrantz River.
These analyses, given in Table VII, show variations in the
amounts of certain constituents, and furnish evidence of the increase
in silica in the clay as the surface is approached. For comparative
purposes', all the analyses have been brought to a water-free basis.
The numbers are those used in Professor Mountain's original paper.
Analyses 7 to g, and 14 to 21 were summated, as they were taken
from approximately the same depth below the surface, even though
from different horizons in the weathered shale.
From the only trench, in which incidentally a ferricrete lens is
exposed, giving a section of the Dwyka tillite clay under the silcrete,
two general samples of the clay were taken. These. were analysed
for comparison with the Lower Dwyka shale clays. B was in contact
with the overlying silcrete and A came from 1 ft. 6 in. below B.
Samples of silcrete from the base and top (the distance being just
Qver 6 feet) of the formation overlying the Dwyka tillite near the
ferti~rete trench were collected for analysis.
The analyses of these
days and silcretes are given in T:able VIII.
TABLE VII.
ANALYSES OF AN ASCENDING SEQUENCE OF CLAYS, DERIVED FROM LOWER DWYKA SHALE.
To Water-free basis.
~
Summation of
7 to 9,
14 to 21.
13
Black
Shale_
Si0 2 ...
A1 20 3 ...
Fe20 3 ...
CaO ...
MgO ...
K 20
...
...
S03
H 2O+
...
...
...
...
...
...
...
...
'"
...
...
...
...
...
...
...
..
.. ,
.. ,
.. ,
.. ,
...
.. ,
...
,
...
...
...
...
...
...
...
...
10
11
66·22
13·07
8·03
2·42
0·31
2·16
1·20
6·59
68·93
19·31
4·20
0·22
0·36
2·06
0·10
4·80
70·87
20·31
0·05
1·02
0·20
2·37
0·12
5·07
72·28
20·36
0·08
0·25
0·08
1·49
0·07
5·35
100·00
99·98
100·01
99·96
Silcrete Ovedyulg.
...
...
...
...
...
...
...
Si0 2 ...
...
A1 2 0 3 ...
...
Fe20 3 ...
...
CaO ...
...
MgO ...
...
Ti0 2 ...
...
Loss on Ignition
94·72
0·55
1·74
0·63
0·23
2·13
0·34
100·34
(J.J.F.)
Analysis 13 of somewhat weathered shale from a well about half mile east of the Osman Kiln Stack, 50 yards north of the railway.
Analysis 10 comes from the railway cutting running east-west, 200 yards north-east of the Osman Kiln Stack, north of the railway.
Analysis 11 comes from north-north-east of No. 10, about 200 yard~ north of the railway line on the flats at the head of the valley.
TABLE VIII.
ANALYSES OF CLAYS DERIVED FROM THE DWYKA TILLITE AND OF SILCRETE ON THE TILLITE.
To water-free basis.
Dwyka Tillite Clay.
Si0 2
Al 20 3
Fe20 3
CaO
MgO
Na 20
K 20
H 2OH 20 +
Ti0 2
S03
MnO
...
...
A.
65·91
12·70
3·15
1·43
0·59
0·32
1·06
6·26
6·95
0-93
0·62
0·03
B.
68·45
13·84
3·78
0·88
0·09
0·47
0·38
6·29
4·07
1·29
0·35
trace
99·95
99·89
Si0 2
Al 20 3 (+MnO and
Ti0 2 )
Fe20 3
CaO
MgO
2
Na
K 00} as K 20
2
S03
H 2 O+
A.
70·30
14·56
3·36
1·52
0·63
C.
B.
73·07
1·57
16·14
4·03
0·92
0·10
1·01
0·66
7·40
0·37
4·34
100·00
99·98
Si0 2
Al 20 3
Fe20 a
CaO
MgO
Alkalis as K 20
H 2OH 2O+
Ti0 2
92·02
1·51
2·38
1·10
0·05
0·60
0·19
1·30
1·36
D.
95·03
0·09
2·04
0·43
0·04
0·42
0·06
0·40
2·17
100-42
100·68
2·63
2·65
S.G.
Analysis A. From the ferricrete trench about 1 ft. 6 in. below the Silcrete.
Analysis B. From the ferricrete trench in contact with the overlying Silcrete.
Analyses C and D. From base and top of silcrete respectively, on Dwyka Tillite from Cradock Road near the ferricrete trench.
C and D by J.J.F.)
(Analyses A, B,
22
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
From both tables of analyses the following facts are evident:(a) The amounts of silica, alumina and titania increase as the
surface is approached, except in the silcretes, where the
al umina decreases.
(b) The amounts of alkalis, magnesia, calcium oxide and
sulphate decrease in general.
(c) In the case of ferric oxide, a general decrease is noted in
the Dwyka shale clay and in silcrete overlying tillite. In
the tillite clay, however, there is a slight increase,' which
may be explained by the following reaction:
F'e2(S04)3+3Ca(OH)2 ~ 2Fe(OHL+3CaS0 4
Subsequent leaching, no doubt, was responsible for the
removal of the calcium sulphate.
Further examination of the analvses reveals that the removal of
soluble salts increases the percentag~s of silica and alumina in the
clays by residual concentration, and that as the surface is approached
the ratio of alumina to silica increases. This may be attributed to
the decomposition of complex aluminium silicates by meteoric waters,
resulting in the formation of clay minerals and the freeing of silica,
which is then dissolved by the liberated carbonates, now in solution.
2KAISi 3 0 s + 2H 2 0 + CO 2 ~ H 4 A1 2Si 2 0 9 + 4Si0 2 + K 2 C0 3
Orthoclase
Kaolin
The free silica is then transported to the uppermost layers, and is
deposited in the pore spaces of the incoherent material of the subsoil. It is likely that most of the alumina remains in the clays as
clay minerals. Silica is easily transported by natural waters, but
alumina is not to any appreciable extent. A warm climate is necessary
for the decomposition of clay minerals to free silica and form bauxite,
etc., but in the present area no deposits occur which could suggest
that appreciable decomposition of clay minerals has taken place.
Mountain (16) has given a norm for analyses 18-21, and it is
interesting to compare norms for the two analyses of Dwyka tillite
clay which are given in Table VIII in this paper.
ANALYSES
18-21.
.DWYKA TILLITE CLAYS.
A.
Quartz
Kaolin
Muscovite
52
31
17
Ferric oxide, etc.
100
...
53·2
24·0
13·3
10·2
B.
54·1
32·6
8·5
5·4
100·7
100·6
.From the nbove comparison the general increase in silica and clay
minerals is seen. 'rhe ra t'io of clay mineral to <]Uf:l.rt7. increases and
the complex aluminium Bjlicatefl decreasf. The decomposition of the
GRAHAMSTOWN SURFACE QUARTZITES.
23
complex aluminium silicates into clay minerals and silica, and the
removal of the latter, thereby increasing the amount of clay mineral,
is thus further demonstrated.
The undoubted replacement of felspar by silica is seen in thin
sections of Dwyka tillite. In sands passive cementation by silica takes
place, but in soils and clays not only does cementation occur, but also
a large percentage of replacement.
Amongst phenomena intimately associated with processes of
metasomatism and replacement are those of capillarity and precipitation. Solutions and colloidal dispersions moving in capillary openings
react with the material through which they pass. Capillarity is, therefore, a fundamental phenomenon in the deposition of materials at the
surface. Progressive precipitation will eventually narrow the capillary
openings in rocks, so that precipitation may take place by reaction of
electrolytes only, w hen sols cannot easily penetrate.
When the upward migration of solutions due to capillarity brings
dissolved materials into the sub-soil and soil where different conditions
exist-these conditions being the presence of oxygenated water, humic
acids and probably, in the case of the area under consideration, a large
concentration of sodium chloride (owing to the nearness of the sea
at the time of formation of the silcretes). all diffusing downwards, as
well as an increase in pH (13a )-precipitation will take place.
In the case of true solutions, the precipitated salts may be brought
into solution again and taken into the soil and sub-soil by cold
descending waters. In the case of sols, -after having risen by capillary
action they may be coagulated irreversibly in the sub-soil, soil or at
the surface.
According to Lovering (13b) cold descending waters dissolve
silica owing to the presence of salts such as halite, epsomite, sodium
bicarbonate, magnesium bicarbonate, etc., derived from the soil, in
solution.
:Moore and Maynard (15a) state that "silica in solution
in cold natural waters is transported as a colloid provided the concentration does not exceed 25 parts per million, but if the concentration is higher, it is possible for a small percentage to be transported
as alkaline silicate."
Lovering states (13a) that" much of the silica, which is precipitated a.s secondary qua.rtz or chalcedony ... probably comes out of
solution when acid waters rich in silica are made alkaline in the
presence of electrolytes." Coagulation is only effected by ionised
solutions of a neutral or alkaline reaction, whereas acid solutions
hinder this process. The action of dissolved salts in dilute solutions
on silica appears to be one of solution, but when solutions become
concentrated, coagulation of the dissolved silica sol takes p!ace.
l\iagnesium bicarbonate is one of t,he best solvents of quartz (13c),
and one of the best coagulants of non-dilute colloidal silica is sodium
chloride (15c).
In a region of alternating wet and dry seasons, the water table
fluctuates; in the dry season the colloids are precipitated; the dis-
:d4
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
solved salts will in part be removed at the surface, so that the descending waters in the wet season will be more dilute, and if the pH
remains unchanged, the precipitated colloids will be unaffected, (31)
remaining as a gel in the capillaries of the sub-soil and soil.
The ideas given above can be used to solve the problem of
~ilicification at the surface of a region of perfect peneplanation. The
extensive weathering of the shale to produce the clays is evidence of
prolonged leaching, and the leached soluble constituents are those
which are most efficient in dissolving silica and thus causing its
transport.
Silicification is certainly not proceeding at the present. time;
indeed, it is very likely that, in view of the erosion and diss~ction of
the peneplain, climatic conditions have changed .. It is possible, too,
that the removal of barriers and the consequent cessation of peneplana~
tion have resulted in the probable complete absence of chemical leaching. The rejuvenation of streams at the commencement of erosion
no d.oubt· affected the drainage and also the water table level.
The progressive silicification of the silcrete itself is seen from
the two analyses given in Table VIII. The deposition of silica from
the colloidal state was due no doubt to the change of conditions where
upward moving s.olutions met .other conditions in the sub-soil and soil
as before mentioned, and the idea that the " zone" of silicification
was not at the surface, but in the 80il and sub-soil, or more likely
where soil and sub-soil meet, is worthy of serious consideration. After
th8 deposition of the silica as a gel, the greater percentage of the
soluble salts was removed into the soil and so out of the picture.
From a consideration of the above facts, the main chemical
features of the formation of the silcretes are
(a) The removal of soluble constituents in the clays, resulting
in the residual concentration of silica and alumina.
(b) The decomposition of the complex aluminium silicates into
clay minerals and silica, the latter being removed in a
colloidal state, resulting in an increase in the ratio of
alumina to silica in the clays.
(c) The replacement of the clay minerals by colloidal silica,
together with its deposit,ion in the sub-soil, giving rise to
the formation of the silcrete.
In the silcretes overlying highly arenaceous rocks, the only
process which takes place is one in which the authigenic 'silicain the
silcrete is derived from the underlying rocks by solutions containing
dissolved salts.
Storz (30) believes that the agent of silicification was alkaline
silicates in the colloidal condition and not ordinary silica hydrosol,
but according to Moore and Maynar.d (15b), who cite ,F'. W. Clarke:
" It is not possible for the silicates of the alkalies to, exist as such at
dilutions found in most natural waters."
A possibiiity, therefore,
exists that if the conditions in the Kalahari were such that colloidal
25
GRAIIAMSTOWN SURFACE QUARTZITES.
alkaline 8ilicates were responsible for the processes of silicification,
then on these grounds it appears that the solutions operating in the
formation of the Kalahari silcretes were more concentrated than those
operating in the coastal silcretes, if, as we consider, colloidal silica
was responsible for silicification on the coastal peneplain.
A ferricrete lens occurring in the silcrete is an indication of a change
of a local nature in environmental conditions. In swampy areas natural
water contains more iron in colloidal solution than does other surface
water, and this iron by the flocculating powers of vegetable m.atter
of bacterial nature may be precipitated as ferric hydroxide. 1£ the
ferricrete be considered a result of a small swamp area on the peneplain
during the period of silqrete formation, then it is possible that organic
agencies were operative. There is, however, no evidence in the rock
at present to indicate that such agencies were present. Rogers records
the formation of ironstone at Heidelberg in Karroo beds: " . . . after
the exceptionally wet years, the formation of the ironstone was
conspicuously active
(22b).
The mutual precipitation of two sols may also be responsible for
the coagulation of mixed silica and ferric oxide hydrosols. As ferric
oxide hydrosol is positive, it may, on coming into contact with silica
hydrosol which is negative, cause mutual precipitation. Gels tend
to adsorb salts until a chemical compound develops. Such compounds
may form by the precipitation of hydrosols by electrolytes, when the
constituents may be in any proportion. 1£, however, two sols mutually
precipitate each other, a compound with simple molecular proportions
results. According to a new theory of Thomas and Johnson, cited by
1\foore and lVIaynard (15d), " the mutual precipitation of ferric oxide
and silica hydrosols is due to the removal of the peptising agents by
chemical action between them.
The amounts of authigenic silica and ferric oxide in the ferricrete
can be obtained from the following analysis:I I
II
Water-free basis.
Si0 2
A120 a
Fe 2 0 a
CaO
MgO
K 20
Na 2 0
H 2 O+
H 2 OTi0 2
MnO
51·29
9·84
25·73
0·89
0·09
0·79
0·13
7·88
2·06
1·34
trace
52·35
10·04
26·25
0·91
0·09
0·81
0·13
8·04
100·04
99·99
1·37
(J.J.F.)
Specific Gravity: 2·84
1£ alumina and ferric oxide are considered as ferric oxide simply,
because alumina behaves similarly to ferric oxide, then the ratio of
26
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
total silica to ferric oxide in the ferricrete is 51·29/35·57. Moore and
Maynard (15e) cite the reaction mixture used by Thomas and J ohnson, which resulted in complete precipitation, as containing 172 parts
of ferric oxide and 117 parts of silica in a million. In calculating the
ratio of silica to ferric oxide in the ferricrete matrix, it is, however,
necessary to neglect that amount of silica present as quartz grains, of
which there are a fair number. It was not possible to separate the
quartz grains from the silica present in the matrix by chemical
methods, and consequently the Shand Integrating Stage was resorted
to. The ratio of quartz grains to matrix was found to be 22/78, from
thin sections examined.
Because grains in thin section are truncated, it is obvious that
their true diameters and thus percentage weights in the rock are not
correctly represented. It has, however, been determined (8) that
by increasing the observed readings on grains by i, fairly accurate
results may be obtained. F'or the purpose of the present calculation,
this procedure was adopted.
The analysis was brought to a water-free basis, and as the specific
gravities of the rock and of quartz are known, the percentages by
weight of allogenic quartz and matrix (the latter composed of authigenic silica, ferric oxide and alumina in the hydrated condition, and
the other constituents in the rock shown in the chemical analysis)
were calculated, using the corrected volume ratio determined
previously.
The amount of allogenic quartz found to be 30·79 per cent. by
weight was subtracted from the amount of silica as given in the
chemical analysis. The ratio of authigenic silica to the iron and
alumina in the matrix is, therefore, 21·56/36,·29, corresponding to a
percentage ratio of 37·27/6,2·73. The ratio of silica to ferric oxide in
the mixture used by Thomas and Johnson was 40·49/59'.51.
Considering the possible errors in a calculation such as we have
given above, the agreement between the two results is an indication
of the mutual precipitation of the silica and ferric oxide hydrosols in
the formation of the ferricrete.
On examining the analyses given in tbis paper, it will be noticed
that as a rule a fair amount of titanium dioxide is present in the
silcretes; most of this titania can be accounted for by the presence of
ilmenite. leucoxene, detribal grains of rutile and also needles of
secondarv rutile. These have been observed in thin section and also
in the heavy residues. The secondary rutile needles so common in
clays are found only in the silcrete overlying such formations, and
no doubt they were present in the claysi before silicification.
The remainder of the titania present is probably adsorbed in the
matrix, in structures which are characteristic of coagulation from the
colloidal state (19). These so-called colloform structures are, as seen
in thin section, a cloudy grey to cream in colour. Occasionally they
are a dull red-brown colour due to adsorbed iron. They are as a rule
either spherical or spiral, and are composed of a series of concentric
GRAHAMS'I'OWN SURFACE QUARTZITES.
27
circles or isolated whorls. In the sections examined they are small,
the largest are about 0·6, mm. in diameter, though this is not· general.
A macroscopic occurrence of colloform structure several inches across
was seen on a block of silcrete on Dwyka tillite at the ferricrete
trench.
Colloform structures have only been noticed to any great extent
in silcrete over clayey and sandy clay formations, but in interstices
between large grains in silcrete over Witteberg, minute structures have
been noted, probably formed in the replacement of clayey material
which originally filled the interstices.
Owing to the larger volume of chalcedonic material in silcretes
overlying clays, it is apparent that in the gel there was an opportunity
£01' the rhythmic diffusion and subsequent deposition in bands of other
amorphous minerals to take place, thus exhibiting the well-known
Liesegang phenomenon.
The colloform structures are evidence of this rhythmic deposition,
and they are composed of bands and whorls rich in leucoxene and ferric
hydroxide in the chalcedony.
In the more " compact " silcretes formed by the silicification of
sandy sub-soils, the space filled by silica gel was much smaller and
the structures did not develop to any great extent.
When silica hydrosol coagulates, it contracts in volume and
cracks and small voids appeal'; on ageing this gel converts to opal,
then chalcedony and occasionally fibrous quartz. Shrinkage of the
gel in the silcretes took place in certain directions, resulting in
cavities vaguely orientated, and due to the fact that after the greater
proportion of the silica was deposited in and partly filled the voids,
the remaining solution was dilute and slowly crystallised in the minute
geodes.
OTHER SUR.F'ACE DE·POSITS.
Other types of surface deposits have been observed on the
Grahamstown peneplain, but as the present work is predominantly
concerned with the silcretes, the former will be merely discussed in
brief.
Occurring over a large portion of the silcretes is a. surface
brecci.a, composed of silcrete set in a ferruginous matrix. This
deposit is on the average a foot in thickness, but occasionally it is
two feet thick.
Scree Deposits occur on ledges cut in the slopes of the range of
mountains and its poorts, which lies to the south of Grahamstown.
The screes are composed of large angular boulders, small angular
pebbles and coarse sand set in a matrix of normal to somewhat
ferruginous silcrete. In F'eatherstone's Kloof and other localities some
very slightly rounded boulders of Witteberg quartzite are seen, which
suggest that some of the screes are in part fluviatile. The screes are
28
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
generally very thin, as they are subject to particularly active modern
erosion. This is borne out by the absence of any similar deposits on
other ledges at the same height.
The ledges probably represent halting stages in the erosion which
took place prior to the levelling of the main peneplain, and after their
formation, detritus from the mountains accumulated on them. The
interspaces between the boulders were then filled with sand and
silicification followed.
The Ferricrete. Exposed in a trench to the NNW of the brickfields and occurring in a block of silcrete somewhat lower than the
main peneplwin, is a lens o£ ferricrete about 1 ft. 6 in. in maximum
thickness and 20 ft. long. Th:is rock is of considerable interest when
the climatic and chemical changes which took pla,ce during the formation of the, surface deposits are considered, but it is of small extent
and of no great importance ~n the distribution of the surface deposits.
It :is overlain and underlain by massive silcrete, of the type developed
on the Dwyka tillite.
In hand specimen it has a concretionary appearance .and is composed of small fragments of silcrete set in a ferruginous matrix. A~
seen in thin section, the rock is composed of irregular sub angular and
occasionally rounded grains of quartz up to' 0·6 x 0·8 mm. in size,
set in a semi-opaque reddish to yellow-brown matrix of hydrated iron
F'elspar is
oxides. The colour of the matrL,{ suggests limonite.
entirely absent. Pisolites averaging 1·5 mm. in diameter and sometimes up to 7 mm. are present. A thin border of dark ferruginous
material is usually found around the margins of the pisolites, which
contain fewer quartz grains than the rest of the matrix. Bands of
varying shades of reddish-brown often traverse the matrix.
At the trench in which the ferricrete occurs, the iron-rich breccia
mentioned above is well exposed, and is to a certain extent similar
t,o the older ferricrete. The amount of silcrete in the ferricrete is
considerably less than in the breccia, and consequently it is more
ferruginous than the latter.
DISCUSSION OF OBSERVATIONS.
The formation of the silcretes has been considered from the
chemical point of view, and an outline of the probable conditions of
the deposition of the authigenic silica has been given. Further light
on the origin of the silcretes is thrown by the petrographic studies
already presented.
The silcretes, in thin section, show differences in the nature and
relative amounts of allogenic to authigenic materials. These differences, which will become apparent from inspection of the camera
lucida drawings, show that the silcretes in a remarkable way correspond with the types of rock they overlie.
For example, the silcretes overlying highly siliceous arenaceous
rocks such as the Witteberg quartzite are compact with little authi-
GRAHAMSTOWN SURFACE QUARTZITES.
29
genic silica whereas those overlying the tillite have a larger percentage of authigenic material. The tillite is, strictly speaking, not a
highly siliceous rock-an analysiR (In) of clay from weathered tillite
shows 73·48 per cent. silica, and this may be even greater than the
silica content of unweathered tillite.
The Lower Dwyka shale is also not highly siliceous, and the
silcrete resting on it also has more authigenic material than the
silcrete on the Witteberg. In the case of the former silcrl3te, however, there are large quartz granules and pebbles which are derived
from quartz veins in the clay, and to a slight extent from the tillite.
In the case of silcrete on Bokkeveld, large grains of quartz (there are
numerous quartz veins in the Bokkeveld) and rounded fragments of
quartzite are present-these are undeniably derived from the Bokkeveld sandstone.
An interesting piece of evidence showing that, the rocks weathered
in situ to produce the residual incoherent sediments which subsequently silicified, is the presence in silcrete resting on tillite of
numerous pebbles of vein quartz. Although sporadic pebbles of vein
quartz are found in silcrete resting on Witteberg, the amount present
in silcrete on tillite is greater. It can be seen in the field that the
tillite contains almost as many quartz veins as the Witteberg. Most
p~bbles in the tillite are granites and other easily weathering rocks,
and it is not very likely that they would be found in the silcrete on
the tillite. Chert is rather rare in the tillite, but a pebble of banded
chert which can be matched exactly with specimens collected from
the tillite has been found "in the silcrete overlying tillite in the quarry
on the Cradock road.
Silcrete on Witteberg contains smaller grains on the average than
Witteberg, possibly due tb the breaking up of grains on weathering in
the sub-soil and soil. When examined in thin section, most grains
in the Witteberg silcrete furthermore show evidence of a fair amount
of solution, whereas the grains in silcrete on the other formations are
hardly embayed at all. The almost total absence of abundant
secondary enlarged grains in silcrete might possibly be attributed to
the rapid coagulation of the silica hydrosol.
As has been described previously, all the silcretes are composed
of undoubted detrital grains separated to varying degrees (depending
on the nature of the underlying formations) by secondary silica usually
containing adsorbed material of the silt and clay grades. It is obvious
that prior to silicification the interspaces must have been filled with
som,e ma,teriaL This was undoubtedly soil or clay, and confirms the
theory that the silcrete was formed by the silicification and partial
replacem.ent of the clay fraction of soils of varying sand content.
In the section succeeding the Tables of Heavy Residues, it has
been noted that, in the case of l,he relatively homogeneous formations, the Bokkeveld, the Witteberg and the Lower Dwyka shale, the
bulk of the detrital fraction of the overlying silcretes was derived
directly from the immediately underlying rocks. In this connection
30
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
tourmaline is particularly significant in view of its comparatively low
specific gravity (3-04). If any considerable migration of material of
the sand grade had taken place, no agreement between the nature and
amount of tourmaline in any particular silcrete and that found in
those rocks underlying would have been noted.
The case of the Dwyka tillite and its overlying silcrete residues
is certainly at first sight rather strange. The absence of garnet, in
particular, in the silcrete examined is surprising. When the problem
is considered, however, the variation in the residues examined is not
fl.ltogetber inexplicable. The tillite is very. variable in composition,
and this has. accordingly been reflected by the silcrete which formed
on it. Unfortunately, t,ime' would not permit of a, detailed examination
of numerous tillite and silcrete residues to be made.
Mechanical analyses of the sand fraction of Lower Dwyka shale
clay and of material of the sand grade of the overlying silcrete, the
latter obtained by ::treal measurement on several weathered sa,mples,
fohowed an almost identical result.
Lower Dwyka
Shale.
Greater than 1 mm.
·5-1,0
'25- ·5
·125- ·25
Very fine sand
o
Silcrete
Overlying.
o
0·25
5·0
19·75
75·00
2·0
8·0
18·0
72·0
100·00
100·0
This close correspondence and the remarkable resemblance
between the heavy residues suggest that it would not be unduly
hazardous to assume that the major portion of the sand fraction of
the Lower Dwyka shale silcrete was derived by weathering from the
underlying sandy clays.
On the basis of the chemical analyses, it also seems. extremely
probable that the authigenic, as well as the allogenic fraction of the
silcretes, was derived from the underlying formations. Admixture of
solutions near the inter-formational boundaries must, however, have
taken place. :Mountain (16) has suggested that the authigenic silica
in the silcrete overlying Lower Dwyka shale came mainly from the
hills of Witteberg quartzite, which rose out of the peneplain, and
that there seemed to be. no evidence that any of the silica which he
observed to be more concentrated in the upper layers of the clay,
was derived from the shales.
A slight amount of material from the tillite appears to have
contributed, but not the Witteberg quartzite to any extent, as might
have been expected. Perhaps this is not surprising, since the
Mountain Drive ridge was quite a distance away and an anticlinal
GRAHAMS'I'OWN SURFACE QUAR'rZITES.
31
l'idge peneplaned at approximately tLe same level as the base of the
silcrete on the Lower Dwyka shale intervened.
Assuming then that all the sand grains were derived from the
Lower Dwylm shale, then to form 30' feet of silcrete, of which approximately 50 per cent. is composed of material of the sand grade, it can
be simply calculated that approximately 200 feet of shale containing
8 per cent. of material of the sand grade must have been weathered.
Even this figure may be quite low, as the clay examined is
certainly more sandy than the shale itself; also the wind may have
influenced the accumulation to a certain extent by removing lighter
grains and depositing them elsewhere.
The matrix of the silcrete agrees in refractive index with
chalcedony, and there does not appear to be any opal present. The
silica gel on ageing doubtless formed opal, but in the' course of time
this has become, changed to the more stable form of crypto-crystalline
chalcedony. Chalcedony may, however, have been deposited directly,
as experiments have shown that from acid solutions this is possible.
From a cons!ideraticm of all the evidence ava,iluble, both field and
chemical, the tenta,tive. conelusion has been reached that. alkaline
waters are those responsible for tihe coagulation and formation of
opal. Certainly siliceous sinter, which is opaline, is deposited by
alkaline waters.
In the Kalahari it seems possible that the secondary silica which
replaces limestone travels in acid solutions (which are more common
in nature). These solutions are made alkaline by the limestone when
they replace it and form opal. Where the Kalahari sands and other
non-calcareous rocks are silicified, opal seems to be the exception and
not the rule. In the area under consideration, therefore, the solutions
carrying silica were acidic.
The relative percentages of minerals in the heavy residues seem
to remain constant as far as could be detected from the base to the
top of silcrete on tillite, which was the only one examined in this
manner. While there is a slight decrease from 0·014 to 0·008 in the
percentage of residue from base to top, the chemical analyses show
an increase in the amount of titanium dioxide from 1·36 to 2·17 per
cent. (ferric oxide, however, decreases slightly from 2·38 to 2·04).
The extra amount of titanium dioxide in the silcrete from the top is
most certainly present in colloform structures, which were observed
to be mor:e prevalent in the upper portions of the silcrete.
It is deemed advisable to give short accounts of the silcretes
examined from other areas at this point.
In thin section the "silcrete conglomerate" from Honey
Clough in the Gamtoos Valley is seen to contain numerous wellrounded pebbles of silcrete from 2 to 3 mm. in diameter in a younger
silcrete. In these pebbles there is a larger percentage of quartz grains
than matrix. These grains are subunguIal' and angular, having an
average diameter of 0·02 mm. The matrix of the pebbles is somewhat more iron-stained than the younger matrix, in which there is
32
TRANSACTIONS OF 'fHE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
an abundance of extremely minute quartz grains and a few composite
quartzite grains. The nature of this silcrete which overlies pre-Cape
phyllites is essentially similar to that overlying Lower Dwyka shale.
Silcrete overlying the Table Mountain series from Montagu Pass
was also examined. The striking feature of this silcrete is the large
size of the quartz grains, which have an average diameter of 0·5 mm.
Some grains are even 0·5 x 1·0 mm. in size. The large quartz grains
are angular to slightly rounded, and only the smaller grains show any
signs of solution effects. This point is significant, because although'
the Table Mountain sandstone is essentially similar to the highly
siliceous Witteberg, it does contain a fair proportion of argillaceous
and felspathic material, which could contribute secondary silica and
thus leave the quartz grains hardly affected. In the Witteberg, however, hardly any argillaceous material was available, and the secondary
silica necessary for the formation of silcrete had to be derived mainly'
from the quartz grains in the rock, and from solutions permeating
from the neighbouring Lower Dwyka shale.
The silcrete on the Table Mountain series also contains a few
pebbles of a fine-grained silcrete composed of minute quartz grains
set in a dark opaque clayey matrix. The grains in the Table Mountain sandstone are larger than those in the Witteberg quartzite, and
the comparison between silcrete on the former and silcrete on the
latter is additional proof that each silcrete is made up of allogenic
and authigenic material derived mostly from the formation on which
it lies.
Practically an clays contain secondary rutile, as has been pointed
out previously, and these needles are to be seen in the silcretes overlying such deposits. In the Riversdale district silcretes overlie leached
clays derived from the Bokkeveld; which there, is definitely more shaly
than at Grahamstown.
These silcretes contain abundant rutile
needles. Colloform structures are also present in this silcrete, and
they indicate derivation from silica gel-a characteristic of silcrete on
clayey formations.
An occurrence of silcrete on dolerite is described by Adamson (1).
The silcrete contains land flora preserved in situ. The matrix is
chalcedonic. He considers it very likely. owing to the presence of
roots in the rock that it was an original sub-soil.
The residues
obtained from this rock showed no admixture from other rocks, being
composed of minerals usually found in dolerites (see Table VI).
The silcrete at Kentani rests on dolerite and clay also overlying
dolerite. ~rhe clay is 30 to 40 feet thick, and contains small crystals
of quartz (26). It is also, therefore, probable that the clay was
derived from the dolerite from which silica has been leached to form
the overlying silcrete.
A white crust of chalcedonic silica which forms on saline muds
after wet weather has been recorded from Klein Brak River (20).
The presence of a large concentration of sodium chloride, as has been
pointed out previously, will precipitate silica from the colloidal state,
GRAHAMSTOWN SURFACE QUARTZITES.
33
and the fact that, the Klein Brak deposit is so near to the sea suggests
that sodium chloride is responsible for the. precipitation of the silica
at the surface, thus bearing out the experimental evidence quoted in
the chemical considerations.
In all the exposures examined, "silcrete conglomerate" is
dominantly present in the lower portions of the silcrete formations.
This indicates that there were in entirely separated localities at least
two periods of silicification. The fo:r;mation of the conglomerate, in
view of the widely distributed nature of this rock, is probably due to
varying climatic conditions. If the silcrete formed mostly in the subsoil, with :lny period of soil erosion the rock would break down and
form ultimately a pebbly sub-soil and pebble-free soil. When the soil
had been restored and a second climate of fluctuating water table
re-established, silicification recommenced, the pebbly sub-soil forming
a " silcrete conglomerate" and the soil a normal silcrete.
CLASSIFICATIONS OF
SURF~CE
DEPOSITS.
Rogers and Schwarz (24) considered that three distinct types of
surface deposits grading into each other could be recognised on the
coastal peneplain.
(1) The GenadendaZ Typle of Ironstone gravels, which are composed of large rr'able Mountain sandstone boulders cemented by a
ferruginous matrix.
(2) The Napier Type, which has a matrix similar to the Genadendal type-does not contain large inclus1Wns. It is a black loose
sandy ironstone andconta,ins small fragments of quartz and slate.
(3) The Zwartklip Type, of which there are a variety of modifications, is described as a limno- or locular quartzite or burrstone.
It is cream in colour, silieeous and contains winding passages. It
also contains fragments of vein quartz. In its inner portions
seiconda,ry silica has been deposited, and often a vitreous quartzite
has been formed, whose derivation from the locular quartzite can be
Been in t,he hand specimen.
Passarge (17) classified the surface deposits of the Kalahari as
follows : (1) EingekieseUe Gesteine, in which original grains are cemented
by seconda,ry silica.
(2) Verkieselte Gesteine, in :which carbona,te rocks have been
replaced by silica.
(3) Ka17-cige Gesteine (Calcretes), rocks with calcareous cement.
(4) Laterite.
More recently another classifictttion has been formulated by
Storz (30) in which to the elassification of Passarge he adds
(1) Rocks produced by the deposition of silica gel as an in~
dependent deposit in cavities and fissures Of on the suria,ce.
34
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
(2) DUl'chkieselte Gesteine which differ fro-m the eingekieselte
gesteine in size of grain; that is, the silicification of fine-grained rocks,
in which infilled pores are minute, as in clays.
As a result of the present investigation the silicified rocks on the
Grahamstown peneplain may be divided into:(1) Silicified Screes-these are compo-sed of angular to subangular pebbles and blocks of Witteberg quartzite cemented with
ordinary silcrete.
(2) Silicified Crusts--the weathered crusts of the "\Vitteberg and
Bokkeveld series are often silicified.
This type of silicification is
essentially similar to that described from Heidelberg by Rogers (22a),
and also to that described from Rhodesia by Ma.cGregor (14).
(3) Silicified Sandy Sub-soils.
(4) Silicified Sandy Clays.
To complete the above classification" we may add the following
two types of silicified material, not found around Grahamstown:(5) Silicified Sands-i. e., certain Kalahari silcretes and the
occurrence of a silicified sand dune at Bellville Junction (9).
(6) Silicified Clays-silcretes of the type overlying Bokkeveld
shale in the Riversdale district. In the clay, derived from the shale,
not only has new material been introduced (Durchkieselung) but the
clay minerals have been replaced by secondary silica as well.
Types 3, 4, 5 and 6 are here regarded as the true silcretes.
A gradation from ordinary silcrete (type 3) to brecciated Witteberg quartzite cemented by silcrete is common. Of the types of
surface deposits recognised by Rogers and Schwarz, the locular
quartzite resembles our true silcrete and the ironstone gravel finds
its counterpart in the silicified screes.
Of StQlrz's classification, the eingekieselte gesteine are represented
by types 3, 4 and 5, and the durchkieselte gesteine are similar to our
type 6.
It is 01 i;nterest to mention at this point that the "duricrust ,.
of Australia described by Woolno'llgh (32) is essentially similar to
the surface deposits of Southern Rhodesia. His classification is into
three groups, the division being made on chemical grounds.
(1) Aluminou,s and jeT1'ugin )U8 deposit8.
(2) Siliceous depo8its.
(3) Calcareous and magnesian deposit8.
f
THE AGE OF THE SILCRETES.
Little direct evidence is available which would enable the age
silcretes on the Grahamstown peneplain to be determined, even
proronged searches for fossils having proved abortive.
ot the
GRAHAMSTOWN SURFACE QUARTZITES.
35
Silcretes similar to those occurring at Grahamstown, and probably
of comparable age, have been described from Kentani Hill (26). In
a. hand speeim.en of silcrete from near Kentani, presented to the Cape
Geological Commission, silicified Ohara seeds and gastropod shells
were identified. lVIore recently Adamson has described an assemblage
of fossil flora occurring in a similar silcrete from Fort Grey near East
London (1). These he regards as of Tertiary age.
Wybergh (33) has stated that " the absence in the Bredasdorp
conglomerates of pebbles of surface quartzites is another point of
difficulty. . .. It can only be surmised that at the time when the
Bredasdorp rocks were being deposited . . . the quartzites had not
yet been formed . . . . " Hogers (21), in contributing to the discussion
on this paper, stated: " !<,'ragments of these quartzites have not been
recorded from the conglomerates of the Bredasdorp beds, so the latter
were presumably formed before or during the silicification of the surface of the old peneplain."
1£, as has been suggested, the silcret,es. formed in the sub-soil,
then, even if they were· older than the Alexandria system
(which is correlated with the Bredasdorp formation, both being of
Tertiary age), they could not be eroded because of a soil covering.
More recently Haughton (lIb) has suggested a correlation of the
silcretes of the coastal plain in the vicinity of Grahamstown with the
Alexandria system; this correlation being based on "observations
that on farms on the northern side of the Bushmans River .
silcretes seem to grade insensibly on their southern edges into the
limestone. "
The silcrete and Alexandria formation at this locality undoubtedly
rest in close proximity on the same peneplain, the margin being at a
height of 1,300 feet approximately, but it is problematical whether
the Grahamstown peneplain being at a higher elevation (2,000' feet)
is of the same ago.
Irhe fad that coastal peneplains which are oIten capped with
surface deposits have been cut across slightly tilted Uitenhage series
(Lower Cretaceous) fixes the age of the formation of the peneplains
as undoubted post Lower Cretaceous.
The tilting of the Lower Cretaceous beds is regarded by du Toit
as having taken place during the Middle Cretaceous (8). This suggests
that the cutting of the peneplains took place in post Middle Cretaceous
time.
In Grahamstown, as has been indicated, even such small streams
as the Blaauwkrantz have cut, by hea,dward erosion, valleys passing
through 30 feet of hard silcrete into the Lower Dwyka shales. This
dissection must have occupied a very considerable period subsequent
to the formation of the silcretes. Storz (30) has concluded that the
silicification of the Namib commenced in the Cretaceous, and may
36
TRANSACTIONS OF THE GEOtO(}ICAL SOCIETY OF SOUTH AFRicA.
have reached its maximum after the deposition of the Pomona formation, which has been regarded as pre-Middle Eocene. 2
On the basis of this evidence, it seems probable that the Grahams.
town peneplain is of late Cretaceous to early T'ertiary age, and that its
subsequent sub-surface silicification took place during the early and
middle Tertiary.
ECONOMIC ASPECTS.
As silcretes are composed almost entirely of crystalline or cryptocrystalline silica, they are of considerable interest from a refractory
point of view. Rocks of high silica content are essential for silica
brick manufacture. Specifications have already been enumerated in
connection with South African rocks suitable for silica brick production (2a).
The Findlings Quartzite much used in Germany bears a striking
resemblance to the silcretes described in the present work. Reference
to the camera lucid a drawings will show that the average Grahamstown silcrete is similar in appearance and grain size to the typical
Findlings Quartzite. The following is a description of the German
rock:" The Erratic Block quartzites ... consist of fresh wa.ter deposits
of Tertiary origin. . .. Under the microscope the grains of quartz
are ... very small, with rounded edges, and are distributed uniformly
through an amorphous mass. . .. They appear to have been formed
of minute grains of sand cemented together by a siliceous jelly, which
gradually hardened and formed a siliceous cement. The chief
accessory minerals are zircon and tourmaline. Rutile and muscovite,
which are found in almost all other quartzites, are not found in these
erratic quart,zites " (29).
Thin sections of the Findlings Quartzite were examined, and in
all cases thin needles of rutile, which have probably formed as an
end mineral in the alteration of ilmenite, were noted. Occasionally,
too, small patches of silicified quartzite occur in the rock.
The presence of rutile needles and the large volume of authigenic
silica obviously derived from the precipitation of a sol indicate that
Findlings Quartzite is a silicified residual sandy clay. The silcretes
bearing the closest resemblance to the F'indlings Quartzite are those
occurring on the Lower Dwyka shales and the Bokkeveld series, both
of which are predominantly argillaceous formations. In consequence
there is no hesitation in regarding the origin of Findlings Quartzite
as similar to that of the coastal silcretes of South Africa, in contra.diction of a recent stl1.tement (2b).
2 BEETZ, W., cited by HAUGHTON
S. H.: "The Occurrence of Upper
Cretaceous Marine Fossils near Bogeniels." Trans. Roy. Soc. S.Ajr., p. 365,
Vol. XVIII, 1930.
GRAHAMSTOW~
37
SURFACE) QUARTZITE)S.
Silcrete overlying clays has been described from Australia (3)
A partial analysis shows a high silica content., viz.:Si0 2 •••
AJ 20 a
Fe 2 0 a •••
H 2 0...
98·56
0·85
0·15
0·32
99·88
It is eminently suitable for refractories, and is used in silica brick
manufacture. It also contains silicified wood. It is overlain by a
Tertiary basalt flow, and the author considers that " the conversion
of the sandstones into hard flinty quartzite is a metamorphic effect
of flows of Tertiary basalt."
In the light ot the present work on South African silcretes, it
seems likely that the silicification of the Australian rock is due to
solutions of recent age from the weathered basalt, or more likely from
the underlying clay, and that the processes of silicification are similar
to those which operated in the formation of the South African
silcretes.
Other points of similarity between the Australian and South
African silcretes is the nearness to thel sea of both deposits, and their
limited thicknesses. In the Australian rock the thickness varies
between two and fifteen feet. In South Africa the average thickness
of the silcrete is twenty feet.
Of the silcretes occurring on the Grahamstown peneplain, that
which occurs on the tillite may not prove suitable for silica brick
manufacture, owing to the occasional presence of fragments of fresh
and altered felspar. The silcrete occurring on the Dwyka shales and
Witteberg quartzite would possibly be more suitable.
The following representative analyses are of Grahamstown
silcretes from different localities; an analysis of Findlings Quartzite
is added for comparison. No analysis of silcrete overlying the Bokkeveld series was made owing to the extremely limited nature of the
deposit : 1.
Si0 2
Al 2 0 s
...
...
...
...
...
...
...
...
...
...
...
...
Fe 20 a
CaO
MgO
Ti0 2
Loss on Ignition ...
1.
2.
3.
4.
5.
6.
...
...
...
...
...
...
...
2.
3.
4.
5.
6.
92·95
0·26
3·60
0·40
trace
2·77
0·55
94·38
0·33
3·13
0·21
0·02
1·60
0·78
94·72
0·55
1·74
0·63
0·23
2·13
0·34
91·68
0·34
5·04
0·55
0·24
2·45
0·35
97·09
0·48
0·94
0·10
trace
1·51
0·26
97·7
0·39
0·49
0·07
0·10
1·53
0·35
100·53
100·45
100·34
100·65
100·38
100·63
Silcrete overlying Dwyka tillite from quarry on Cradock Road.
Silcrete overlying DwykatilJite from near ferricrete trench.
Silcrete overlying Lower Dwyka Shale from above the brickfields valley.
Silcrete overlying Witteberg Quartzite from Featherstone's Kloof.
Silcrete overlying Witteberg Quartzite from south of brickfields valley.
Findling8 Quartzite (2c).
(Analyses 1 to 5 by J.J.F.)
38
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTII AFRICA.
Althol1gh the silica content is slightly low, the fairly high percentage of ferric oxide lin the Grahamstown s:ilcretes might be an
advantage in the thermal and mechanical properties of silica brick
(2d). The high titania, content might also be an advantage in reducing
spalling tendency and increasing resistance to thermal shock (2e).
An artificial process of silicification has been evolved (4), in
which sand is hardened into a compact, rock, or a friable rock is made
more compact. This process is of considerable interest from the
mining point of view. The method simulates that which has taken
place in nature·-Waterglass is introduced into the sand under pressure,
after which calcium chloride is forced in.
The waterglass sets to a gel immediately, the water in excess is
squeezed out and the gel hardens to produce a tough rock. The
grains of sand used in Germany appear to be between 0'·75 and
1·0 mm. in diameter, small grains are not used because porosity is
reduced. The photomicrographs of the artificial quartzite, which
appears to have about 10 per cent. of authigenic m.aterial, show a
" rock" which is similar to the silicified Witteberg crusts,. and which
IS actuany used as a building stone.
No doubt an application of this process to the preparation of
suitable material for silica brick manufacture is possible, but the
natural sources of siliceous materials and particularly silcrete would
probably be more economical.
CONCLUSIONS AND R:gSUM:g.
During the last stages of the Cretaceous period, an extensive
peneplain commenced to develop where Grahamstown is now situated,
and probably attained perfection just prior to or during the early
Tertiarv.
Thve northern slope of the Mountain Drive ridge bears a narrow
inclined terrace carrying silcrete resting directly on the Witteberg
quartzite. This terrace is about 140' feet higher than the main peneplain with which it is in part continuous. It is not impossible to
conceive of this representing a marine bench mark, and that the
peneplain is, therefore, of marine origin. On the other hand, the
alternative theory is that the peneplain has formed as a result of a
prolonged period of base levelling by rivers.
All the original deposits, whether the peneplain is of fluviatile
or marine origin, seem to have been removed by unknown agencies,
probably aeolian, and the peneplain subsequently weathered for a
long period under sub-aerial conditions, resulting in the formation
of incoherent residual deposits. Practically no admixture of these
deposits or addition of "foreign" material took place, and they,
therefore, closely correspond in composition with the immediately
underlying rocks.
The most inland occurrence of the Tertiary marine Alexandria
formation in the Eastern Province at Sandflats lies at an elevation
GRAHAMSTOWN SURFACE QUARTZITES.
39
of 1,150 feet above sea-level. This formation represents a succession
of dune and beach deposits formed on a receding shore (lla). When
the first of these beds were laid down, the Alexandria sea must surely
have lapped around the base of Woest Hill to the east of Featherstone's Kloo£. The main Grahamstown peneplain was then, therefore,
very near the sea.
Under these conditions the sub-soils of the residual deposits on
the main peneplain became silicified by the upward migration of acidic
colloidal solutions of silica derived by the quiescent leaching of the
underlying rocks. The coagulation of this silica in the sub-soils was
doubtless aided by the presence in the soil of downward migrating
solutions rich in sodium chloride, which was transported by the wind
from the nearbv sea.
The result of this silIcification was the formation of a layer of
silcrete, resting on residual clays where it overlies argillaceous formations, or directly on sandstones Hnd quartzites in the case of the
arenaceous. The process of formation of these silcretes in the subsoil was interrupted on several occasion{ During these "intersilicification" periods, the first formed sl~cretes often broke up by
weathering, and when silicification again commenced, the resulting
product was in part a " silcrete conglomerate." These alternating
periods of silicification and non-silicification are probably to be
ascribed to climatic variations-it has been shown that a seasonal
fluctuating water table is essential for the formation of the silcretes;
if the climate became torrid (in which case the water table would
sink below the sub-soils) or too moist and uniform (when the water
table would not fluctuate), silcrete formation would end.
As the Alexandria formation was gradually built up, sea-level
receded. This recession was not definitely uniform, but seems to have
been accompanied by halting stages, which permitted of the formation
of other peneplains at lower levels. On these, too, conditions of poor
drainage and fluctuating water table seem to have prevailed and
silcretes formed. Only fragments of these once extensive peneplains
now survive, but their former distribution can be traced in the field.
Of these lower peneplains, it, is quite probable that some are of marine
origin.
The climate necessary for the formation of the Kalahari silcrete
has been postulated by Storz as torrid, which permitted of the
weathering of minerals. The occasional presence of fresh felspar in
the silcrete on Dwyka tillite and the considerations already described
seem to indicate that the climate during the formation of the Eastern
Province silcretes was temperate.
As the level of the sea continued to fall, the system of drainage
in the vicinity of Grahamstown became rejuvenated, and vigorous
headward erosion by the Blaauwkrantz River system gradually
resulted in the peneplains being dissected and a drainage system
being impressed upon them.. The conditions necessary for the formation of silcretes thus ceased to operate and erosion then set in.
u
40
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
We have seen fit as a result of the differences in degree of " compactness " of the allogenic fraction of the surface deposits to divide
the true silcretes into two groups: (a) Those formed by the silicification
of sands and clayey sands derived from highly siliceous arenaceous
rocks, and (b) those formed by the replacement and silicification of
sandy clays and clays produced by weathering from formations
dominantly argillaceous.
ACKNOWLEDGJ\1ENTS.
Our thanks are due to Profes-sor E., D. :Mountain for his unfailing
interest, criticism, collecting of specimens and for supplying certain
thin sections otherwise unobtainabie; also tor his contributions to
this paper. We are grateful to Dr. Haughton and Professor Stanley
for permission to complete laboratory work.
One of us (L,.E,.K.) is indebted to Mr. F. C. Partridge for
demonstrating some of the technique which he has evolved for the
extraction and examination of heavy residues; we thank him also for
the loan of thin sections. Further, we must thank Mr. L. W.
Vermeulen for the interest he took in certain of the chemical analyses,
and Mr. F. May, of Hankey, and Miss M. Murray, of Rhodes
University College, for collecting specimens.
We thank Dr. Haughton and Mr. E. Mendelssohn for a critical
reading of the typescript.
BIBLIOGRAPHY.
(1) ADAMSON, R. S. "Fossil Plants from Fort Grey near East London." Ann.
S. Afr. Mus., Vol. XXXI, Part I, 1934.
(2) (a) BOSAZZA, V. L. "Notes on South African Materials for Silica Refractories." Tran8. Geol. Soc. S. Afr., pp. 465-478. Vol. XXXIX, 1936.
(b)
ibid., p. 474.
(c)
ibid., p. 467.
(d)
ibid., p. 466.
(e)
ibid., p. 468.
(3) BROWN, r. A. "Some Tertiary Formations on the South Coast of New South
Wales." J. Roy. Soc. N.S.W., pp. 387-399. Vol. LIX, 1925.
(4) DIETZ, C. "Kiinstliche Sandverkieselung." Z. Prakt. Geol., pp. 57-61. Vol.
XLI, 1933.
(5) DU TorT, A. L. Rep. Geol. Comm. C.G.H., p. 108, 1910.
(6) DU TOIT, A. L. Explanation of Cape Sheet No. 28 (Pondoland), p. 35.
(7) FRANKEL, J. J. "Geology of a Portion of the Gamtoos Valley." Trans.
Geol. Soc. S. Afr., p. 277. Vol. XXXIX. 1936.
(8) GRATON, L. C., and FRASER, H. J. "Systematic Packing of Spheres."
J. Geol. Chicago. Footnote 39, p. 847. Vol. XLIII, 1935.
(9) HATCH, F. H., and CORSTORPHINE, G. S. "Geology of South Africa." London,
1909, p. 332.
(10) HAUGHTON, S. H.: "The Tertiary Deposits of the South Eastern Districts of
Cape Province." Trans. Geol. Soc. S. Afr. Vol. XXVIII, 1925.
(ll) (a) HAUGHTON S.H. Explanation of Cape Sheet No.9 (Port Elizabeth), p. 28.
(b)
ibid., p. 29.
(c)
ibid., p. 31.
(12) LAMPLUGH, G. W. "The Geology of the Zambesi Basin." Quart. J. Geol.
Soc. Lond., p. 198. Vol. LXIII, 1907.
GRAHAM::;l\()W~'
(13) (a)
SURFACE QUARTZITES.
41
LOVERING, T. S. (a) "The Leaching of Iron Protores, etc." Econ. Geol.
Vol. XVIII, 1923, p. 523.
(b)
ibid. p. 524.
(c)----ihid., p. 530.
(14) MACGREGOR, A. M. "An Advanced Stage in the Process of Sandstone
Silicification." Proc. Rhod. Sci. A88., p. 49. Vol. XII. 1914.
(15) (a) MOORE, E. S., and MAYNARD, J. E. "Solution of Iron and Silica."
Econ. Geol., p. 277. Vol. XXIV, 1929.
(b)
ibid., p. 302.
(c) ----ibid., p. 507.
(d)
ibid., p. 512.
(e)
ibid., p. 513.
(16) MOUNTAIN, E. D. "The Grahamstown Ceramic Industry." S. Afr. J. Sci.,
pp. 135-139. Vol. XXVIII, 1931.
(17) PASSARGE, S. "Die Kalahari," p. 59S. Berlin, 1904. See also KALKOWSKY,
E. "Die V erkieselung der Gesteine in der nordlichen Kalahari. Ah. d.
N aturwi8. Ge8ell8. "I8i8" in Dresden, 190 l.
(IS) POLINARD, ED. "Les Roches essentiellement siliceuses de la region Sandoakafakumba." Ann. Soc. GeoZ. Belg. (Congo Belge), pp. 15-26, 1933.
(19) ROGERS, A. F. "Review of Amorphous Minerals." Econ. Geol. Vol. X,
1915.
(20) ROGERS, A. W. "Raised Beach Deposit near Klein Brak River." Rep.
Geol. Comm. C.G.H., pp. 291-296., 1905.
(21) ROGERS, A. W. Proc. GeoZ. Soc. S. Afr., 1919.
(22) (a) ROGERS, A. W. "The Geology of the Country around Heidelberg.
Govt. Printer, Pretoria, p. 49, 1922.
(b)
i[;1~d., p. 69.
(23) ROGERS, A. W. "Surface Geology of the Kalahari." Trans. Roy. Soc.
S. Afr., pp. 57-SO. Vol. XXIV., 1936.
(24) ROGERS, A. W., and SCHWARZ, E. H. L. "Report on Caledon, Bredasdorp,
etc.," p. 5l.
Rep. Geol. Comm. C.G.H., IS9S.
(25) HOGERS, A. W., and SCHWARZ, E. H. L. "Notes on a Journey from Swellendam to Mossel Bay," p. 19. Rep. Geol. Oomm. C.G.H., 1901.
(26) ROGEItS, A. W., and SCHWAR7., E. H. L. "The Division of Kentani," p. 66.
Rep. Geol. Comm. C.G.H., 1901.
(27) SCHWAR7., E. H. L. "The Coast ledges in the South-West ofthe Cape Colony."
p. 73. Quart. I. Geol. Soc. Lond. Vol. LXII, 1906.
(2S) SOHWARZ, E. H. L. Proc. Geol. Soc. S. Afr., pp. xl-xli, 1925.
(29) SEARLE, A. B. "Sands and Crushed Rocks." Vol. I, p. 143. London, 1923.
(30) STORZ, M. "Die Diamantwiiste Siidwest Afrikas." Vol. II., ch. XXV., 1926.
See also" Die sekundare authigene Kieselsaure etc." Monogr. Geol. Palaeont.
Serie IT, HeUt 4, 1925.
(31) WOOLNOUGH, W. G. "Origin of White Clays, etc." Econ. Geol., p. 887.
Vol. XXIII, 1925.
(32) WOOLNOUGH, W. G. "Influence of Climate and Topography in the formation
and distribution of Products of Weathering." Geol. Mag., pp. 123-132. Vol.
LXVII, 1930.
(33) WYBERGH,W. "The Coastal Limestones of the Cape Province." Trans.
Geol. Soc. S. Afr., pp. 46-47. Vol. XXII, 1919.
The reader is also referred to : (1) "Geology of South Africa," by A. L. du Toit, p. :359. Edinburgh, 1926.
(2) "Colloidal Chemistry and Geology," by R. E. Liesegang.
" Colloidal Minerals," by C. Doelter, in Vol. III of "Colloidal Chemistry,"
edited by J. Alexander. The Chemical Catalogue Co., Inc., 1931.
(3) "Application of Colloid Chemistry to Mineralogy and Petrology," by Alexander
Scott.
Fourth Report on Colloid Chemistry. British Ass. Adv. Sci., 1922.
(4) "The Colloid Chemistry of Minerals and Ore Deposits," by Waldemar Lindgren.
Chapter XVIII in Vol. II of "The Theory and. Application of Colloidal
Behaviour," edited by R. H. Bogue. McGraw-Hill Book Co., Inc., New York,
1924.
42
'l'RANSACTlONS OF 'l'IIE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
EXPLANATION OF PLATES.
PLATE
I.
Photograph and explanatory overlay, of a contour model of the area in the
immediate vicinity of Grahamstown, showing drainage and the distribution of the
silcrete on the underlying formations. Contributed by Professor E. D. Mountain.
PLATE
II.
No.1 Composite Camera Lucida drawing of thin sections of Witteberg quartzite
(A), Dwyka tillite (B), and sand grains from the Lower Dwyka shale (C) and
Bokkeveld shale (D). X 28.
Camera Lucida drawings of thin sections of
No.2 Silcrete overlying Bokkeveld shale.
No.3. Silcrete overlying Witte berg quartzite.
No.4. Silcrete overlying Lower Dwyka shale.
No.5. Silcrete overlying Dwyka tillite.
All X 28.
PLATE
III.
No.6.
Silcrete overlying Table MOlmtain sandstone from Montagu Pass
28. For comparison with No.3 (Plate II).
No.7. Composite Camera Lucida drawing of thin sections showing:
(a) Colloform structure in silcrete overlying Witteberg quartzite. X 84.
(b) "Pasty" bands in silcrete overlying Dwyka tillite. X 10.
(c) Portion of fine-grained wavy band in silcrete overlying Witteberg quartzite. X 84.
.
No.8. Ferricrete. X 28.
No.9. Silcrete overlying Bokkeveld shale, Riversdale District, showing
" silcrete conglomerate" (a) and Colloform structures (b). X 28.
No. 10. Findlings Quartzite. X 28.
X
•
,,
•
•
..
LEGEND
[S1
Bokke-veld Series
lZZ1
Lower" ])wy ka
0
WlffebeY'J
SerIes
m
])w!Jka 7i11,fe
8• • •
Sllcrefe s
PLATE
1.
Shale,
•
1'B.4NS. GE()L. SOO. B.A., VOL. XL ,
PLATE 1.
1'B.4NS. GE()L. SOO. B.A., VOL. XL ,
PLATE 1.
[Sl
Bok..ke. \Ie Id Series
D
W,.ffebeYJ
0
e
••
Ser,es
[ZZJ
Lowe.,..]Jwyk,a
m
])w!Jka 7illlfe
Sllcrefes
PLATE 1.
Shale,