Geology. - "On the Composition and the XenolitAs oj thè Lava­dome oftlLe Galunggung". (West-Java). By Prof. H. A. BROUWI<llL (Communicated by Prof. G. A. F. MOLENGHAAlfF).

(Communicated at the meeting of November 27, 1\-)20).

During an eruption of Ibe Galunggnng, which eommeneed on the 18th of .luly 1918 and pl'oduced only an ineonsiderable fall of ashes, a lavadome was formed in the emter, w hieh fOl'med on the 20t1l of JuIy an islet in thc cmterlake I), arld whieh gmdually ~I'ew 80 large that the entire lake disappeared. On the ti th of August only the N. W. pad of t.he Wal'irang-emtor was eovcred withwater').

On a visit 10 the Galnnggnng-crater Dl'. W. VAN BlliMMgU~N, on my request, searched for xenoliths in the roeks of the dome, in order to aseertain w hother any erystalli?'jation had taken place in the magma under the crater gimilar to thaI in the dome of the Ruang (Sangi lslands) 3). The collection transmitted to me through the "Headoffiee of the Mining Depal'tment" cOlnprises flllmerous samples of lava from the dome with fine-erystalline 10 coarse-grained homoeogeneous xenoliths, whieh will be deseribed lower down.

l'he lava of the dome. All the rocks examined are brownish-rcd, por'ous bypersthene­

allgiteandesites with phenoerysts of zonary plagioclases, among whieh frequently nal'l'ow basic and more acid zones oeem alternately, so that the marginal zone, even in the case of mal'kedly zonary struc­tU1'O, is of ten only little mOre acid than the central part; also the suceeeding zones differ but little as to basieity.

Carlsbad twins oeCllr; in sections of the symmetrieal .zone we deterrnined that, on an avel'age, the composition of the plagioelases is like that of hytownite Ab .. Ahw Inelusions, among which some of ore and of a glassy substance are genel'ally few in numbsr; a zonary al'mngement, tbe inclusions being limited to certain zones, is sometimes met with. 'I'he hypel'sthene- and augite phenocl'ysts

1) R. D. M. VERBEEK et R. FENNEMA. Geologische Beschrijving van Java en Madoera. n.

2) B. G. ESCHER. De uitbarsting van den Goenoeng Galoenggoeng. De Taak 12 Oct. 1918 pp. 126-127 en Mededeeling namens B. G. ESCHER door G. A. F. MOLENGRAAFF. Versl. Geol. Sectie. Geol. Miinb. Gen. Il. Oct. 1919.

3) H. A. BROUWER. Crystallisation and Resorption in the magma of the volcano Ruang. (Sangi Island). Proc. Kon. Akad. v. Wetenseh. Vol. XXIII, p. 561.

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are of ten accllrnulated; sometirnes they are fonnd together with plagioclase- and larger ore crystals. Wherc the pyroxenes are con­tiguous to groundrnass they are cornrnonly endrcled by a narl'OW zone of o1'e, which is lacking w here they are iJl contact witb plagioclases. Appal'ently Ibis is a ehemical exchange between the phenocrysts of pyroxene and the still Iiquid part of the enclosing magma. In some samples of the dome resorbed bl'own arnphiboles we re found; ho wever, Ihey exhibited no idiomol'pbous erystalform and we venture to assume that they are not crystals formed in the dome but fmgments transported by tbe rising magma. Some of them may be fragments of the same cl'ystaIIization produets, from whieh the amphibole-rich homoeogoneous xenoliths originate.

Tho groundmass of Iho dome-roek is rieh in glass and eontains lath-shaped plagioclase and gmins or skeleton-shaped illdividuals of Ol'e, while pyl'oxene has not (or only to a small degree) erystallized in this grOllnd-mass.

The :cenoliths of the domeroclc. Among them we distinguisb the following types:. 1. medium-grained, oeeasionally porphyric xenoliths, eonsisting of

plagioclase and amphibole with a srnall qllantity of a more or less de vi trified glass.

2. medium-, to coarse-grained xenoliths, made up of plagioclase, amphibole with little pyl'oxone and sornetirnes a little olivine. Glass OCCUl'S also in these xenoliths.

3. medium-, 10 coarse·grained, sometimes porphyric xenoliths witb plagioelase, (little olivinê), amphibole and much augite and hyper­sthene. 'I'he olivine was se en only in some xenoliths, Ol'e sometimes occurs in a small quantity outside tbe l'esol'ption-rims of the amphibole. Tbe rolative qllantity of amphibole, augite, alld bypersthene is variabIe. All xenoliths, in whieh the number of pyroxenos is not very smal!, have been included here. Glass with micl'olites was observed ollly in some xenolith and in a very small quantity.

4. porphyric xenoliths with phenocl'ysts of plagioelase in a fine­grained grollndmass in two generations with plagioclase, pyroxene and ore. Lal'ger pyroxene-crystals do not oceUL' in the xenoliths, but are accllmulated in a small marginal zone against the enelosing rock.

5. tine-gmined xenoliths, made up of plagioelase, augite, hyper­sthene and littIe ore. lVluch glass with micl'olites is found in some samples between thc othel' minel'als.

6. xenoliths of older andesites, sorne of thern bearing arnphibole, others devoid of amphibole.

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The {renoliths 1IIentioned sub 1 (Plate; fig. 1 and 2) at'e first of all chat'aetel'ir,ed by plagioclases, which are distillgnished fl'om tho&e of the phenocl'ysts of the cnclosing rodrs by the almost total abseuee of the frequent altel'l1ation of more basiC'. alld more acid ?'ones. 'l'be crystals, often r,OTHny and wilh a bl'oad basic cenlntl part are ehiefJy eomposed of basic bytownite, the marginal zone is more acid.

Tbe \~mphit){)le is stl'ongly pleochl'oitie, from a bl'ownish red to light-yellow, and invariably shows a l'esol'pLion-l'im, of ten only narrow in many xenoliths anc! sometimes entil'ely absent VI' here the cl'ystals are contiguous to plagioclase, while it is often weil developed where the amphibole is in contaet with the glass-rieh mass witl! mierolites. TbeRe nal'l'ow I'esol'ption l'irns are eomposed of a black mass of 01'0.

It is obviollS (hat in part the amphibole has crystallir,ed later than the plagioelases, which form idiomorphous erystals and then guard agaim;1; resorpLioll th at portion of the amphibole witlt which they are in eontaet. In tbe xenoliths with more strongly resol'bed amphi­bole, there oeeu!' entirely l'esol'bed cl'ystals, whieh can only be l'ecognized as original amphibole by their crystalfonn. In tbe cl'ystals that are partIJ unaltel'ed, tbe l'esol'ption-l'im consists only of a maas of ore or a marginal zone of Ol'e, sepal'ated from the illtaet part of the cl'ystal by an il'l'eg'ularly shaped pYl'oxene-l'ich zone, whieb is aometimes laeking and whieh sometimes oecul'S mixen with un­moditied amphibolc. Out of the l'csol'ptioll rims larger el'ystals of me do not oeem' in the xenoliths.

Some of Ihe xenoliths witlt stl'ongly l'esol'bed amphiboles present a, porphlyrie stl'ueture, the groundrnass, which coniains large, more or less idiomorphons amphibole cl'ystals, consisting of pJagioelase. Between these smaller plagioelase erystals, as iJl the non-p0l'phyric xenoliths with larger plag'ioelases, more or less devitl'ified glass is found. Plagioelase-microlites and me-skeletons are easy to distingllish in this mass; only a smal! l!lunber of pyroxene microlites are distinguishable; the devit.rification is sometimes eomplete.

'l'he limit bet ween the xcnoliths and the enelosing lava is always sneh that the lava has adapted itself 1.0 the slJapes of the xenolith. Tbe crystal faces of the plagioelases and amphiboles have l'eached full development at t.he mal'gin of the xenoliths so thaI the boundal'Y line with the la\'a proeeeds irregularly. Also the enclasped g'lassj' mass shows that the minerals had not been pel'fectly eI'ystallized, when thc xenoliths were taken up in tLle enelosing magma, so they were still molten to a cet'tain extent and may therefore be eonsidored as an almost perfectly cl'ystallized erllst on the magma, w hieh was effused from a larger depth and has produced the dome. 'l'he

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gl'oundmass of the lava of tbe dome and the devitrified glass of the xenoliths have then cl'ystallized almost simultaneonsly.

The .xenoliths mentioned sub 2 do not diffe1' mnelJ fl'om thosejnst cleseribed; they at'e all chal'actel'Ïzed by a small amOllllt of Pj't'ox~ne, while also a small amount of olivine was fonnd iJl some. 'l'he amphiboles show a nal'l'ow l'esOl'plion rim, whieb eonttiins mllcll 01'0, mixed with pyroxene (pl'ineipally aug·ite).

Veins of tbe same eompos.ition as the l'esOl'ptioll rim are fOllnd in Ihe mOt'e centl'al part of tbe erystals, whieh are lal'ger than those of llte ot.hel' minel'als, whieh preseJlt an il'l'egulal' ontline and may enelose all the otl1el' eom ponen ts en ti rel)' Ol' pal'tly, also oli v ine, if it OCCllrs in the xenolitbs. The olivine [ll'esents ronnded s!tapes without distinet el'ystal races and bas been altered 10 some deg;'ee into a brownisb red Ol' black suustance, l'iehin iron. The glass-beal'ing rnuss is rieh in laths of plag'ioelase alld also eontains many pyl'oxene­microlites. The glass is vel'ydul'k and the devitrified portion is appal'ently l'ich in iron; O1'e bm'dly shows itself in separate gl'ains.

The a:enulitlts with more pyro,'l:ene, mentioned sub ~ (PI. fig. 3) are distingnished from tbe pl'eceding chiefly by the deel'ease of Lhe amonnt of amphiuole and the incl'ease of pyroxene. Val'ions stl'uetUl'es o~elll'. Pyl'oxeno (augite as weIl as hypersthene) ocelHS oecasionaJly wtth a few larger amphibole-, and plagioelase-erystals in a finel' erystalline mixture, eOllsisting mainly of plagioclase with !ittle p.'P·oxene. The pyroxene (above all '[ he augi te) t hen ex bi bi ts si mi lar skeleton-like structul'es to tbose of the amphibole, and (hen incloses, just like the last.-named minal'al, nllrnel'OIiS plagioelase el',Ystals, whieh also penetrate into the augites witb idiornol'phous erystal form, 80

that these are one of tbe last cl'yslallisation products ofthe xenolitb. IJl eonneetion with this the smalleJ' plagioelases are entil'ely free from enelosed dark minerals, th~ large!' nearly 80. Tt is these xenoJiths tbat eontain olivine wiLb rounded shapes and mostly enelosed by the amphibole. Devitrified glass oeeuJ's in small quantity.

In other xenoliths aJso bearing oeeasionally olivine, fol' the rest !ittJe different from the othe1's, the pyroxenes me chiefly I'cstricted jo thc fine·erystalline bulk pf tbe xenolith, while only few larger cl''ystals occu!' with some of partly l'csol'bed amphibole and of plagioelase. A pOl,tion of the xenoliths of this gl'OUp displays the nonnal, medium grained stl'ueture withont largel' crystals; here we find evidenee of the posteriOl' erystallizatiol1 of the ampbiboles, becallse they oceul' in skeleton shaped erystals, which oeeasionally enelose tlte pyl'oxene. The different stages of l'esorption of tbe amphibole, mentiohed al ready fol' thc xenoliths desel'ibed sub 1,

12:-38

were also met with here. No oli"ine was found in the samples of the mediumgrained xenoliths examined.

l'he xenoliths mentioned sub 4, found only in some specimens, are free from amphibole, just as those mentioned sub 3. They are characterized by a I'omarkable structure, large plagioclase-phenocrysts lying in a groundmass, consisting of smaller L'l'ystals of plagioclase, few ol'ecrystals and very fcw of augite and hypersthene. The latter in theil' tUI'Tl are sUl'l'ounded by a fine-cl'ystalline mixture of the same mineraIs, whose constituents - with the exception of the smal! plagioclases- occllr in a large nnmber as inelUflÎons in the plagior.lases of tbc first and te second generation (PI. fig. 4). Zonary structure does not occur with these plagioelases or only in a smal! measure and without the alternation of basic and more acid zones, which distinguish them from those of t.he enclosing lava. Tbe line-cl'ystalline groundmass is almost entirely absent in a narrow marginal zone of the xenolith where th is is contiguolls to the enelosing lava. Here we see a mixture of plagioclase, lik~ those oceurring everywhere in the xenolith as small­sized phenocrysts, together with the angite-, hypel'sthene-, and o1'e­crystals, which are seen only in srnall number in the central parts of the xenoliths as small phenoerysts. It appears then that pyroxene and O1'e al'e accumulated in the mal'ginal zone. The stl'uctures deseribed heretofOl'o point to tho ,fact that the cI'ystallization of the xenoliths was still to take placd for the most part, when they bad al ready been taken up in tbe enclosing lava. In an early stage tbe plagioclases, the pyl'oxenes, and the ore-cl'ystals of the secOlld gene­ration have crystallized. The latter two have accurnulated in the marg'inal zone of the xenolith. That tbe L'l'ystallizatioJl of the plagio­elases was the first to be finished here, is proved by the idiomorph­ons shape of thc crystals relative to the pyl'oxenes in the marginal zone and the enclosure of plagioclase by pyroxones, which occurs froquently here. In the central parts we see tltat the erystallization of the o1'e and of tbe pyroxenc of the fine-crystalline gl'oundmass had already begun dllt'ing the erystallization of the plagioclases, some of whieh have grown into larger phenocrysts. Then followed the ultimate crystallization of the fine-erystalline groundmass, in which occurs the plag'ioclase in I'onnded shapes, which points to a erystallization about simnltaneous with (hat of tbe pyl'oxene, Tbe plagioclases of thc marginal zones are pOOl' in or destitllte of in­elusions alld seem, therefore, to have crystallized bef 01'0 those of the contml part of the xenolith, or the crys!al!ization of thefine­crystalline gl'onndmass has taken plaee in tho rnarginal zone later

H, A. BROUWER: ..

A. BROUWER: "On theComposition and the XenoHths of the Lavadome of the Galunggung". (West-Java).

Fig. 1. Magnification X 24. / nicols. Xenolith with plagioclase and amphibole. The amphibole is slightly resorbed. The enclasped glass­bearing mass has partly disappeared in the section.

Fig. 3. Magnification X 42. 1/ nicols. Xenolith with plagioclase and pyroxene. In this xenolith amphibole occurs rarely. It is not visible in the preparatton, while the enclasped glassmass with microlites has. partly disappeared.

Proceedings Royal Acad. Amsterdam. Vol. XXIII.

Fig. 2. Magn. X 24. X ni cols. Ibid. Fig. 1.

Fig. 4. Magn. X 42. X nicols. Phenocryst with plagioclase with numerous inclusions of pyroxene and ore. Porphyric xenolith with phenocrysts of plagioclase in a finely crystalline ground mass in two generations with plagioclase, pyroxene and ore.

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and in a smaller degree, 110 that tbe quantity of it is small tbere. Tbe boundary between the xenolith and the enelosing lava, of whieh the latter has adapted itself to the shapes of tbe el'ystals in the xenolitb, proves moreon~l' tbat the xenolith has not been ellelosod at greater depth in a solid state, but that il !Jas beon taken up in the onelosing lava as a partly erystallized matis.

Tlte :cenoliths mentionecl sub 5 eOllsist of plagioelase, augite and hypersthene with a few ore-erystals, the interspaeos being filled up with val'ious qW1fltities of a partly devitrified, dal'k substanee, against whieh feldspar-rniel'olites stand out shal'ply outlined. Tbe plagio­elases and the pyl'oxenes present mostly idiornol'phous outlines (espe­eially the plagioelases); in eonlradistinetion to that ofthe xelloliths IIms far deseribed, the stl'ueture of' tho plagioclase is zonal'y with froquent alternation of more acid and basie zones, like those of the phenocrysts in the enelosing lava of thc dome rnentioned above.

The xenoliths o} oldel' andesites mentionod sub 6, display differ­enees as regal'ds mineralogieal eomposition and sf,ructure. Jn some of them a fow plagioclase pbenocl'ysts oceur in a groundmass, consisting of' plagioclase and pyroxene with ore.

In others ampbibole was observed, oeeasionally as phenoeryst in similar roeks to those montioned above, sometimes in aUl'eoles round ol'eel'ystals, oeeul'l'ing porphyrically wilb plagioclase in a dark gIass­bearing groundmass. Ft'equently tho microscopie aspect changes considel'ably, e. g. as to tbe amount of are and as l'egards the eolou!' of the groundmass, while tbe rocks give an impression of being modified throngh contaetmetamorphism, in which pl'oeess recrystalli· zations havo taken placo. The porphyric plagioclasos have been stl'ongly eroded by the elear mixture of which the present ground­mass consisis; wo then suppose tbe groundmass /0 have been entil'ely recrystallized and the phenoerysts only in their marginal zone. The aUI'eoles of amphibole l'ound ol'e-cl'ystals in a partly devitrified groundmass find an expJanation in the assllmption that what haf) taken place hel'e is just tbe revorse of what happened wit,h the resorption of amplJibole. Tbe enelosed andesile fra,gment, heated anow, has been for some time submitled to temperature­and pressure-relations, whieh do not affect amphibole and this minel'al has crystallized inslead of eomponents that otherwise build up resol'ption rims.

Tlte val'ious cl'ystallizations in tlte Oalunggung ma.qma. The boundaries of tbe homoeogeneous xenoliths I'elative 10 the

enelosing lava pl'oves that the xenoliths had not erystallized rom­pletely when tbey wem laken up in the lava. rrho residual magma

f~~

1240

has cl'ystallized as a glass-bearing mass with microlites, just as the grollndmass of the enelosing rocks.

That in many xenolitbs the amphiboles exbibit a l'esorption-l'im whel'e tbey border on lhe glass-twal'ing mass alluded to, points out that the amphibole remained slable down 10 the moment of the el'uption of the lava dome. Aftel' this the pressure in the lava and

'*' the xenoliths decl'eased rapidly, wbich made the amphibole instabJe and augite cOl.l1d cl'ystallize dnring Ihe time when the temperatnre of thc l'esidual mngma feil, and a complete solidification had not yet been effeCLed. rrhis intel'val may have been longer Ol' shorter fol' different pOl'tions of tlle dome, hence the l'esorptioH in amphi­boles of different xenoliths was val'yillg. Already befol'e we accounted 1'01' tbe minel'alogical diffel'cnces between xenoliths anel dome-Java of the RUANG 1) we bave assumed th at during the eruption of thc volcano [he outpouring' magma enclosed fragments of its completely Ol' partly solidified clioritic erust. The same appJies to dome and xenoliths of tbe Galunggung. Tbe oceUl'l'enee of pyl'oxene-poor and pyroxene-l'ieb amphibole-beal'ing xenoJitbs may be arcounted fol' by tbe assumption thai they ol'iginate from zones at various clepths in this rrust. lt depends on thc difference of pl'essme and wmperatul'e of these !.Ones whethel' amphibole only or first pyl'oxene and later, on furthet' cooling, amphibole has cl'ystallized. ') ,

In that case the pyroxene-bearing xenolitbs originate from cleepel' wnes according as tbey are richel' in pyl'oxene, whereas at a gl'eatel' depth witb a higher temperature mueh less cl'ystalline components and only pyroxenes ocent' in the ontpouring magma, whieh does not eontain amphibole as phenocl'ysts.

The ttmphibole-free xenoliths with different structures described above, may have crystallized already bet'Ol'c the el'UptioIl at a great depth, so above tempel'atul'es, at which the amphibole is stabIe, while in that case complete cl'ystallization bas taken place aftel' the el'uptioll had commenced, when the amphibole was not stabie

eitbel', in conneetio!1 with the suddenly modified pressul'e and tem­pel'atllre l'elations. Maybe sorne of these xenolitbs have crystallized at a [JreSSHl'e lower than th at of the stage of stability of tbc arnphibole and may therefol'e belong 10 parts of the magma that have eooled down more slowly, that eould cl'ystallize more Com­pletely along the walls of the vent and were only then earried along by the outpoul'ing; magma,

I) H. A, BROUWER, Crystallizations etc. loc. cit. p .. 665, 2) F. BECKE. Gcsteine des Columbretes, Anhang. Tscherm, Min, u. Petrogr.

Mitt. XVI. 1897. blz, 327 e.v.

Geology. -- "On the A lkalil'ocks of tlte Se1'Ta do Gericino to the nOl'thwest of Rio de Jimeil'o Ilncl the Resemblance between the E1'uptive Rocks of Bmzil ancl t/wse of South-Africa." By Prof. H. A. BuouwmL (Communicared by Prof. G. A, F. MOL ENG RA HF).

(Communicated at thc meeting of January 29, 1921).

On the bonndal'Y between the State of Rio de Janeiro and the Distl'icto Federal lies neal' t,he statioll of Maxambomba of tbe E. F, Central do Brazil, tbe Sel'l'tl do Gericillo 1), extendillg in W.S.W.­N.N.E. direction over a lengLh of' about twellty lcm. and a bl'eadth of abolll eight k.m. It is ehiefly composed of nephelinesyenites like the Tinguft el'llptive province whieh lies about 80 km. farthel' to the Nortb and of whieh the alkali-rocks have been desCl'ibed by GRAIO<'l<' ') and IhRBY 3). As I eould not personally visit the Serra do Gerieino dUl'ing my stay in 13razil, several samples fol' fut·ther investigation were seIlt me by GONZAGA D1<, CAMPOS, Dil'ector of the "Serviço geologico e mineralogieo do Brazil".

Geo logical Survey.

rrhe allmli-rocks constitute an erupti\'e cenh'e amidst tbe old gneisses of t.he mountain-l'ange Sena do Mal', wbich runs parallel to tbc Brazilian Coast. (Fig. 1). We only know thai the gneisses are intruded by alkali-rocks which conseqllently must be younger than the gneisses. No data we re obtained conee1'lling tbe exact geologieltl age.

Coarse-gl'ained, as wel! as dyke-, and effm,ive rocks OCC II 1', juS!

as in other Bl'ltzilian oeCUJ'l'cnees of alkali-rocks. Although near!y the wllOle l'cgion and espeeially the highel'lt parts eonsist of eoarse­grained t'Ocks, the OCClllTenCe of effusive roeks allied to them, still

1) MATHIAS G. m; OLIVEIHA Roxo. Resumé of the preliminar uote on the Gericinó f'l'llptive eentre. Empresa Brasil Editora 1920.

2) Fa. GIUEFF, Minel'alogisch·petrographische Untersuchung von Eleolithsyeniten van del' Sel'm de Tingua. Neues Jahrb. f. Min., Geol. u. Pal. 1887. Ir, blz. 222 e,v.

3) O. E. DEIWY. On nepheline rocks in BraziJ. Quart. JOUl'I1. Geol. Soc. Vol. XLlll. 1887, blz. 457; Vol. XLVI[, 1891, blz. 251.

80 Pl'oeeedings Hoval Acad. Amsterdam. Vol. XXlll.