Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW)
Citation:
H.A. Brouwer & G.A.F. Molengraaff, On Reefcaps, in:
KNAW, Proceedings, 21 II, 1919, Amsterdam, 1919, pp. 816-826
This PDF was made on 24 September 2010, from the 'Digital Library' of the Dutch History of Science Web Center (www.dwc.knaw.nl)
> 'Digital Library > Proceedings of the Royal Netherlands Academy of Arts and Sciences (KNAW), http://www.digitallibrary.nl'
-1-
"On Reefcapi'. By Prof. H. A. BRouwmt. (Communicated by Prof. G. A. Ji'. lVIOUlNGRAAFl!').
Geology. -
(Communicated in the meeting of November 30, 1918).
~
Proof of an llplift of the land relatively to the level of the sea
can, in tropical regions, often be supplied by the pl'esence of npheaved fl'Ïnging reefs. Thus it will be seen that in the eastern part
of the lndian Al'chipelago, whel'e an elevation of the 1'0W of islands
has taken place on a large scale eyel' since the plio-pleistocene ~
period, upheaved fringing reefs, often forming continllolls l'eefcaps
orcut' in most of t.he islands, sometimes to a height of abollt 1300 m.
The rhal'actel' of the latest movements in Ihe curving row of
islands of the sonth-eastel'l1 al'chipelago, wbich l'esulted in 1) tbe
formation of these islands, has been discussed in an earl ier paper ~).
The latest mounlain building is considered to be a l:evival of the
intensi ve yonng-tertial'Y movements, the typical features of the islands
indicating that, just as in the case of the tertiary, also with the
yOllngest mOllntain building the movements are in the direetion of the
"Vorland" whel'eas near the sUl'face moslly faulting is observed. The upheaval of the islands has not been simultaneous, not' eqnally intense
in all places, w hile periods of temporal'y sllbsidence have probably'
in terrupted the genet'al elevatioll since the pIio-pleistorene period.
In this paper we shall try to ascertain whethel' tbere is any ,'elation
between the widely varying' characters of the l'eefcaps we observed,
and the charactel' of the crustal movements. In this eudeavour the
aspect of the movement of the geanticlines dlll'Ïng long periods will
be brought to the front more than has been done heretofore.
The reefs at tlte time of Itltei1' g1'owth.
The g'l'owth of fringing reefs along parts of the coastline may
be prevented by vadous causes, e.g. the lack of asolid substratum,
1) G. A. F. MOLENGRAAFJ<'. On recent crustal movements in the i!.land of Timor
and their bearing on the geological history of the East·lndian Archipelago. Proc.
Kon. Ak. v. Wet. June 29, 1912.
') H. A. BROUWER. Over de bel'gvormende bewegingen in het gebied der boogvormige eilandenreek!.en van het oosteliik gedeelte van den O. I. Archipel. Vers!. ,"
Kon. Ak. v Wet. XXV, p. 768.
-2-
817
impul'ity of the water, and volcallic eruptions. If, hO\"ever, reefs do
develop, the shape of the living reefs depends largelyon the stage
of development of the crustal movemE'ntEi at that moment. In our
d~scussioll we assume a stabie sealevel, because our t'onclusionfl will
also hold for a moving sealeve!. lf the coastline remaills stabIe for
a considerable time or undergoes only slightly horizontal displacements,
hOl'izontal and thick reefs may possibly be formed; slightly vel'tical
movements will perhaps illcrease especially the thickness of the I'eef,
if the mo~ement is a positive Olie, while negative movements will
soon cause the reef to rise above the se a, even thollgh its thiclmess
and exient be only slight yet.
All these phenomena may appeal' simuItaneously at points of the
geanticline remote from each otller, so that already while the matei'ial
is fOl'ming which is to help in the composition of the reefcaps,
considerable diffel'ence in the shape may OCCUI'.
The development of 1'eefcaps.
Aftel' the reef bas risen above the sea, the morphological changes,
which were the combined resuIt of the character of the crustal
movements and the g'l'owth of thc cOl'als, are at an end. Du!'ing Ihe
continued movements the reefs move along cur\'es, whose shapes
vary and are determined by the character of the cl'ustal movements.
These movements may again be alternately vertical or horizontal
and downward, each type manifesting itself during a longel' or
shOl,ter period. The shape of the cm'ves is determined by the evolution of the geanticline on which the reefs were formed. The
reefcap observed by' us is the final pl'oduct of these contimuü and
val'ying movements. The reefs formed at a cerlain epoch on the
sUl'face of the sea, which, initially, were all lying in the same
horizontal level, at'e, in a later stage of development, located in a
plane of il'regular shape, The oldest parts of tbe reefcap have
undergone this change longer than the other portions.
Besidés by the charactel' of the crllstal movements the form of
these l'eefcaps is to a great extent also determined byel'osion.
lnfiuence of 81'ositJn.
In
prise
time
left,
This
areas subject to strong el'osion, it is no matter of surto find that of the portion of a reefcap that has fOl' a long
been elevated above the sea-Ievel, only some remainders are
whereas the youngel' portions still present an unbroken cà p .
will sometimes happen, buL it is, not the l'Ule. There are namely
1'1Sll1g'
.
-3-
818
othet' factors besides time, which govern the inflllence of erosion
on the reefcap, e.g. the nature of the snbstrátnIn on which thereet's have been deposited and the power of I'esistance of the
reefs themsel \'es, lf f he su bstratu m consists of soft rocks, w hich
bring about landslips, while deep valleys are cut in the fOl'mations,
as is the case with a gl'eat pat't of the mesozoic deposits in a numbet'
of islands of the eastern Indian archipelago, the uplifted reef overlying it will soon cl'umble away.
, If thel reef is mel'ely a thin Ct'llst covel'Îng the underground, it
will disappear the soone1'; thick reefs will resist erosion for a considerable time, and wiJl occasionally act as a protective cover over
a soft underground,
As al ready observed, thick reefs will fOt'm in places, where the _
coastline maintains itself for a considerable time, or has undergone
only mOl'e Ol' less horizontal Ol' downwal'd movements. As such they
wi II afterwat'ds constitute pal'ts of the I'eefcap, whel'eas in those
places where the ('oastline has long been exposed to sh'ong negative
movements only a thin reef can be evolved, whielt later on wiII
oecur ns a thin part in the I'eefcap, This pal't is liable to disappear
through el'osion. FOI' it is j nst with these stl'ong Ilegati ve movements
that el'osion often acts very forcibly. so that both factors co-opel'ate
to remove the effects of these mo .... ements from the t'eefcap, With
short negative movements this will be manifested only in a terraced
structure,
lt wiJl, thel'efol'e, fl'equently be seen that, at great heights above
the sea-Ievel, the reefcap is fuHy developed, whereas lowel' down
towat'ds the coasts it has totally disappeared Ol' ,has been preserved
only in detached fl'agments, while on the coasts themselves living
cOI'als are tht'Îving weil. Here we are l'eminded of our investigations
in val'Ïous localities along th~ north coast of the island of Rotti,
along the coasts of Set'mata, Great·Obi, Ceram and Timor, where
the lower elevated reers (if still any have been left.) are fOl' the
gTeater pal't removed by el'osioll, e,g, if they have be,en presel'ved
only on the ridges bet ween the valleys, rlraining' towal'ds the coast.
A long the North coast of Timor the thick I'eef of the Talau básill
abruptly tel'lninates near 13ahbo at a height of ± 610 m. 1), between
Balibo and the actual coast no tl"ace of elevated cOl'al l'aefs is fOllnd,
whereas at the coast living corals are abundant. Hel'e the reef may
have been remo\'ed byerosion, in which pl'ocess the above-mentioned
conditions of 'a rapid erosion must have beeu present, while the
1)
G. A, F,
MOLENGRUFF,
loc, cit., p.
-4-
as.
•
819
more elevated thick I'eef lias been presel'ved, though it had been
longer exposed to the el'oding fOl'ces, If at a higher level a reefcap is lacking', it is impossible to detect whether also here erosion
has been at play, or whether this al'ea has been uplifted from the
sea ever since the beginlling of the cl'ustal movements,
, Tt follows, then, that the influence of el'osion upon the form of
the l'eefcap call be estimated only fOl' the tmct beneath the highest
reefs, which have been left intact byerosion .
rnflttence of laults.
The influence of faults on the form of reefcops is, on the whoIe,
confined to the dislocation of connected parts, which are brought in
"arious positions at differe!lt levels. Faults having played a prominent part in the yOl1ngest crnstal movements in the eastel'Jl archipelago, the fOl'm of the reefcaps may be supposed to bave been
largely affected by them. VERBEEK I) e.g. assumes a fanlt across the
peninsuIa of Hnamual in Sonthwest Cel'am, wh~l'e the ·terl'aces of
cOl'al limestone appeal' sonth of LuIm to the height of 350 m. above
sea level, while the lime more to the north scal'cely reaches 100 m.
In the continuation of this fault we find Hatusna (the eastern side
Piru-bay), Paulohi and Tehoro (on Taluti-bay), which were afflicted
more \'iolently than olher places by the earth- and seaq\lake of 30
September 1899, and also the steep sonth-eastel'l1 coast of Bnru.
When t'aulting takes place iu the neighbomhood of the coasts,
downward as weil as l1pward movements may be observed at shol·t
intervals and the growth of the living' cOl'als mayexert its il1flnelll'e
upon the shape of the fOl'ming l'eefcaps longer than usua!.
The inclination ol the geanticlinal atces.
In discussing the growth of the reefcaps it has been stated thaI
every point of a fOl'ming reef will move along curves of val'ious
shapes. The hOl'izontal cOllIponent of Ihe rate of movement, at a
given moment is the l'esuItant of two directions which are at right
angles_ to eaeh othel', orÎe of whieh coincides with the geanticlinal uxis.
The vertical componellt determÎlles the rising' of the 1'0W of islauds.
The diffel'ence in the l'ate and the direction of t!Je movements at
different points gives rise to the lIlorphological changes of thB SUI'face of the geanticlines of wbich we shall tit'st consider tho.se along
the geanticlinttl axis.
------
1) R. D. M. VER BEEK. Molukken Verslag. Jaarb. v. h. Mijnwezen. Wet. Ged. 1908,
blz. 660. FRENCH: Rapport sur les Moluques.
.
-5-
820
In vit'tue of the changes wbieh tbis axis nndel'goes in a cel'tain
space of time, Ihe ineiinatioll may inm'ease in some places, deerease
iJl otbel's. If we sn ppose the top of the geantielme to I'emain in
the same place, the ditfel'ent poillts of the axis wrlJ, at an incl'éase
of inclination, pel'form movemenLS on eltlter side, which al'e horizontal
towards the top and "eltical in. a downwal·d dil'ection, Also witl! a
slight rise of tbe lop, downwal'd as weil as hol'izontal movements
may OCClll' at a lowel' level along the axis. In Ihib case it wil!, at
a certain stage ot' the evoilltioll of tbe geanticlinal axis, depend on
the height of Ihe sealevel, whether a reef fOl'med at this time will
be moved np Ol' down. The displacement of the reef will invariably
be also in a horizontal direction, fault-movements are left ont of
considel'ation here. Oon versely the h'ansverse eoasts ma}' rise, while
the top of the geantieline iE. deseending at a certain hE'ight of the
sealevel.
Generally the top wiIl not remain in the 5ame plaee, but will be
moved both ill horizontal and in vedical dil'ection; moreovel' the
inclination on eitiJer side of the top wiU not deel'ease or inrrease
in tbe same way. lt does not follow that dlll'ing these Il'regular
movements the tmnsvel'se eoasls WIlt exhibit a similal' behavionr,
and generally , speaking it ma)' be said thai, if the distallee fl'om
the top of the geanticlinal axis to the coastlllle, i.e. in the case of
the larger islands, be snfficiently gr'eat, the vertical componellt of
the dil'ection of the movement at the tops need nol be similarly
dil'ected to tllat at the point& of intersectiolIs of the geanticlinal axis
and the sealevel. This vertical component varies at varions points
along the axis.
The inclined geanticlinal axes in the present-day stage of mountain
building eg. are easily distinguisltable in the islands of the Timor
gl'oup sepamted by btl'aits, and from the above it ma}' be inferred
th'st of all that the I'ate of move ment latterly obsel'\'ed on the trans"el'se coasts of the Jal'gel' islands, is not necessal'ily equal to t he
rate of 1l10vement of the tops, nor need it be of the same dirertion.
This also holds for the eal'liet, stages of the mOllntain building
process. SeeondJy it appears, thel'efore, tha't the height to which a
reef has been llpheaved, by no means depends oniy on the time
elapsed since its fOl'lnation, but on the evolution of the geanticlinal
axis so that reefs of the same age may be elevated to different
heights I) and the highest reefs may- sometimee not be the oldest.
1) As e.g. in Timor. Cf. G. A. F, MOLENGRAAFF, J. c" p. 132 and J. WANNER,
Geologie van West-Timor, Geol. Rundsc~au, IY, 1913, p, ~S9 •..
-6-
821
Asymmetrical Reejcaps,
An asymmetrical developmellt of the geantielmal axis on 'either
side of the highest points yielrls asymmetl'Ïcal reefcaps, This
asymmetry is brought about by the vaI'iable degre€ and direction
of lhe hOl'izontal component of the rate of movement.
We pUl'pose to considet' this development more pal'ticnlarly in a
plane at a right angle with the geallticlinal axis, an instance of
wbich is fOUlld in the island of' Rotti. and the island of Jamdena
of tbe, ,Tenimbergl'oup, Het'e the reefcaps l'iS8 from the northwestern
coast gradually up to the main watersheds of the islands, thence
descending l'apidly towat'ds the sou th-eastern coaRts, Parts of the
reefcaps have disappeared thl'ough el'osion,
The relationship of these asymmetl'ical reefcapR, to certain crustal
movements may be seen from the coincidence of the asymmetrical
stl'uctm'e with marked outwat'd bends of the row of islands, to
whieh the named islands belong 1).
The island of Jamdena lies nearly opposite to a depression m the
Sahul-bank and Ara.f'ura sea; here the geantielinal axis met with
less resistanre anti consequently cOllld be moved more easily than
elsewhel'e in the directioll ofthe"Vorland", The horizontal component of
the rate of movement aL a right angle with the geanticlinal axis may
be considel'ably larger than the vertieal; in conneetion with this
the uprise above the sea will be less, while the unequal size 'of the
horizontal eomponents for varioub points may incJ'ease the asymmetrical forms dUl'ing the development or decl'ease them locally,
FUl'thermore it follows that what bas been said about the development of lhe geanticlinal axes rOl' the transverse coast is also applicable to the moven:tents along the longitudinal coasts and also to
the relative age of reefs, I'aised to different heights, The asymmetrical
l'eefcaps to whose development the horizontal movements have
been highly instrnmental, wil! rifle less high above the sea than the
symmetl'iral, supposing the monntain bllliding fOl'ces to be equal.
In this connection we may compal'e the l'eefs of the island of Timor,
j
N-.-. ___________________ A ________ _
Fig, 1.
I)
H, A,
BROUWER,
1.
C"
p. 770-772,
il-
-7-
I
822
upheaved to about 1300 m., with those of the islands of Rotti and
Jamdena elevated respectively to ± 470 and ± 150 m, which may
be of the same gAological age.
Down wal'd moving longitlldinal coasts sometimes oeenr with rising
islands, Let us take e, g. one of the possible cases in Ihe development of an asymmetrical reefcap, as is shown in Fig. 1. The
points p, A and Q will, in a latel' stage) of development have
l'eached PlJ AI and QI' The sealevel is indicated by the line ..NZ.
The portion AB .of the geanticline rose above the sea in the initial
stage as an island, and may possibly have been covel'ed by a
eontinuons reefeap,
During the development ihto Ihe second stage, discLH,sed by us,
the island wi11 increase in circumference nnd rise higher above the
sea. On the north coast, however, downwal'd movements al'e observed,
while the South eoast is moving upwards.
What was originally the oldest reefcap, AB, will have beeu
transformed and partly disappeal'ed nnder the sea, while the highest
reefs in the serond stage are by no means the oldesl, so that oldel'
reefs will oecul' on a lower level than the younger ones.
In conneetion with the above-mentioned downward movement
along' tlle gently sloping part of the asymmetrical geanticline, we
refer to tlle drowned river valleys, observed by us far inland along
the northwest coast of the island of Jamdena of the Tenimbel'
gronp. 'rhe downward slope ean be only apparent also here, relative to a postglacial rise of the sealevel 1) ').
In contl'adistinction to Timor, RoUi and Jamdena are now also in
theit' central parts covered for the most part with a eontinuous
reefeap. We attribnte th is to the influenee of erosion in eonnection
with the predominating horizontal movements at right angles with
the geanticli.nal axis, Along I.he longitudinal coasls of the last-mentioned islands these movemenls cansed mOl'e resistant reefcaps to
be formed, which moreover wel'e not upl'aised S(} high, 80 that fol'
two reasolls the reefeap was attaeked less while it disappeal'ed
completely (Ol' for the greater part) along the l'apidly l'aised longitndinal roasts of central Timor also for two l'easons.
Of the aspect of asymmetrical reefshields it is often said that one
coast is lifted mOl'e than the one opposite, This assertion, however,
does not assign significance enough to the horizontal component of
1) R. A. DALY. The glacial-contral theory of coral reels, Proc, Amer. Acad. of
Arts and Sciencès. Vol. 51. NO. 4, p, 157. 1915
') G. A, F. MOLENGRAAFF. The coral reef problem and lsostacy. Proc. Kon. Ak.
v. Wet. XIX, NO, 4,
-8-
823
the rate of movement and the continual mOJ'phological change of
the geanticline, which have often been so influential in the development of the reefcaps. In the case iIIustrated in Fig, 1 the reef,
originally formed on the south coast lies, in the next stage, on tbe
northerll slope of the enlal'ged island, so that it is hardly permissible
to speak of a more marked upheaval of the south coast. lt may
even be conceived that also B is sitllated nortb of tbe coastline of
tbe new island, so that in that case the original island is covel'ed
entirely by the sea, while a new island bas ernerged farther sou tiJ.
Elevated reefs of tlte Sel'mata
g7'O'Up.
It bas been 5aid above tbat the reefs, fOl'ITled at a ceJ'tain epoch
iJl the history of mountain bnilding along the coasts of a geanticline,
may pel'fórm varÏolls movements in the snbsequent stages, The rate
as weil as the dil'eclioJl of the movement sornetimes differ conside1'ably at a compaJ'atively short distance. This is clearly illustrated
by the movernents of the reefs in the period of developrnen,t of the
geanticline, in which ollly Hs highest parls erneJ'ge from the sea as
a group of smaller islands, We shaH dweIl more particulal'ly on the
movements of tbe islands of Luang', .Moa, Kisser, and Letti.
According to rny observations in Luang this island, built up
entirely of permian rocks, is together with two islets near the
South-eastern extremity, fringed by a ,'ery broad reef, extending far in
the dil'ection ~f Sermata and also far to the West. Green islels far fl'om
the nOl'th coasl and barren, d J'Y pOl'tions far fl'om the sonth coast,
mal'k the limits in nOt,thern and sontherJl dil'ection; beyond them
the sea floOl' declines rapidly. At ebb-tide paJ't of the reef gets dry,
Luang as weIl ~s the two islets close to it, to the Sonth-east, rise up
steeply fJ'om th is bt'oad reef; no trace of elevated reefs was detected,
so th at pJ'oofs of a period of llpheaval are lacking. The island of
Luang and the two islets lIeal' it, impress us as having ol'iginally
fOl'med one continuous whoIe, and as having been separated by a
positive movement, which mayalso account fOJ' the formation of
thé bl'oad enciJ'cllJlg I'eef, which is bOl'dered here and there by green
islets. Post-glarial llpheaval of the seasul'face renders the sllbsidence
of the land ollly appaJ'ent.
Now let us look at tlle island of Moa, mo~e particularly its
eastern half, For the most part the is1and consisfs of a low, ver)'
. ~road plateau. 9f COl'a:1 limestone, which rises scal'cely m~l'e than
10-20 m, above the sea, and from which in the eastern .pa1't rises
1I
-9-
824
the steep Kerbau mountain 1), which COIISÎSts elltirely of peridotLtes,
Traces of elevated reefs are lacking in the Kel'ban mountain also,
and if tbe eastern part of Moa were a little lower, this region would
present an aspect similar to th at of Luang, Both mountains, the Gg~
Kerbau and the Bt, Mel'ah would then emerge from the sea as two
sepal'ate islands and be entit'ely flinged by a bl'oad reef,
We feel j ustified in assuming that also the lartel' regioll has passed
thl'ough a stage of evolution like that of Lllang at the present day,
and that it has been l'aised above the sea, thl'Ough a 81ight upheaval
aftel' a period of subsidence or a long stationary pel'iod, Ol' according
to DAI.Y throllgh an upheaval aftel' an appal'ent post-glaeial subsidence, By this Ilpheaval also the eastel'n pal't of Moa was Ilnited
to the westel'll part, In the latte I' elevated reefs al'e ,found at a
gl'eater height, so th at the two united islands Ol' gl'OUp of island,s
ha\'e evidently been subject to markedly different movemellts,
The island of Kisser, typified by ita peculiar form, behaved
diffel'ently again. The more or less cireular island, is slll'rounded on
all sides b.v a wall of cOl'al limestone raised in se\'eral (mostl.r live)
tel'l'aces, and bl'oken only by a few nal'l'OW g'ullies, thl'ough whieh
l'i\'ulets flow towal'ds the Se11.. We 8ighted tl1is island only fl'om the
sea; accol'ding to Vl!;RBEEK 2) the elevated reefs in the westel'll part
of the island, near Leweru, reaeh a height of 147 m., whereas the
interioJ', where amphibolite hills prevail, pl'esents peaks ± 240 m,
high, The terl'ared structure of the elevateci reefs points to an eleva- ti on of the island, l'ept'atedly intel'l'Upted by intervals of quiescence
or - considering the thickness of the elevated l'eef's .to !jO m.) of subsidence, by whieh a l'eefcap was formed, strong enough to
resist erosion.
The island of LeW pl'esents quite a different appeal'anee nowadays from that of Kissel', but most likely this island bas also been
encireled by a more Ol' less COlltinuous gil'dIe of fring'ing reefs, of
which at largel' heights occasional remainders wel'e left at 115,129
and 134 m. 3) above sealeve!. Here erosion has demolished the higher
reets almost entü'ely, whieh - on the basis of' what we obsel'ved
about the influenee of el'osion - may hMe something to do with
long and uninterl'upted negative movements,
Many more examples of numel'OUS abnol'mal elevated reefs in the
1) H. A. BROUWER. Geologie van een gedeelte van bet eiland Moa, Jaarb v. b.
Mijnwezen Verhandel, 1916. I, p. 39,
'I R. D, M, VERBEEK, l. C" p. 432.
S) G. A. F. MOLENGRAAFr and H, A. BROUWER. De geologie van bet eiland Letti.
Jaarb, Mijnwezen, 1914, Verb, I, p. 82.
- 10 -
825
neighboUl'ing islands ,could be addllced, but the foregoing sutli('iently
shows th at the evolution of the geantirline has e\'olved during tbe
mountain building process very ir1'egulal' movements at a comparatively shol,t distance.
Tilting lslands.
Among those we ,reckon e.g. tbe island of MisooI, to tbe North
of Oel~!lm"1 Corals are thriving well, as weil on the south- as on the
northcoast, but elevated coral reefs occut' only in the flat northern
part of the island, whel'eas they are lacking entu'ely along the steeper
south coast up to some way past the watershed. The island may
be said to have hlted, if we assume that the south coast has subsided
along the line of a tault at the same time when the no1'tl1 coast
has moved upwards. This ~honld seem to be very likely especially
with the island of, l\'Iisool, because WANNER I) has established the
pr{'senre of a number of faults in the archipelago along the south
('oast bordering on the nOl,th side of the deep sea-basin between
MisooI and New Guinea on the one side aml Oeram on the other.
Howevel', similar reef-fOl"mations mayalso originate in another
\VaJ, where tilting is out of the question, because the movement is
not perfo1'med by the island as such, but because in the initial and
the terminal stage different parts of a developing geanticline present
themselves as islands. In Fig. 1 we have only to look upon the
geanticline in pi BI QI as the initial stage and in PBQ as the terminal
stage. In tbe latter the geanticlir!e bas subsided deeper below the
sea-surface, but on the north coast an npraised reefcap will be
seen. With rising geantielines a similar distl'ibution of the elevated
reefs \Viii also be seen, e.g. in the manner illllstrated in Fig. 2.
A'~-
__
:--_._~-----------~
Fig. 2,
In tbe terminal stage P' A' B' only upraised I'eefs will oeelu' on
tbe north side of the new islànd, viz. between A 'and the north
coast. Then the island is not merel,)' tilted, but exhibits that part
of a rising geanticline, wlJich at a certain time emel'ges ft'om Ihe sea.
1) J. WANNER. Beiträj;e zur geologischen Kennllliss der Insel Misol, Tijdseh.
Kon. Ned. Aardrijksk. Gen. 1910, p. '98.
- 11 -
826
CON C LUS ION S.
1. The parts ot' a reefcap formed dUl'ing negative movement~
may for two l'easons disappeal' l'apidly through erosion. Considel'able
gaps in the development of' a reefcap may, therefore, suggest long
and uninterl'upted negatlve movements.
2. In the rase of geanticlines, raised above the sea over extensive
areas, obsE'rvations a]ong the coast cannot lead to conclusions' about
the movements of the highest points, - also with a stabIe sealevei.
3. The development of the geanticline canses reefs of the same
age to rise to val'ious heights, which sometimes di.ffer considel'ably.
4. The highest parts of a reefcap are not on th at score the
oldest, also when faulting is Jeft out of ronsidel'ation.
5. At the top of a mo\'ing geanticline an island may disappeal'
and yet an island l'emains visible.
6. Islands may tilt or only exhibit the semblance of doing so.
- 12 -