Geology. — “On the Composition and the Xenoliths of the Lava- 
dome of the Galunggung”’. (West-Java). By Prof. H. A. BROUWER. 
(Communicated by Prof. G. A. F. MOrENGRAAFF). 
(Communicated at the meeting of November 27, 1920). 
During an eruption of the Galunggung, which commenced on 
the 18% of July 1918 and produced only an inconsiderable fall of 
ashes, a lavadome was formed in the crater, which formed on the 
20" of July an islet in the eraterlake *), and which gradually grew 
so large that the entire lake disappeared. On the 11'* of August 
only the N. W. part of the Warirang-crater was covered with water’). 
On a visit to the Galunggung-crater Dr. W. van BEMMELEN, on 
my request, searched for xenoliths in the rocks of the dome, in 
order to ascertain whether any crystallization had taken place in 
the magma under the crater similar to that in the dome of the 
Ruang (Sangi Islands) *). The collection transmitted to me through 
the ‘Headoffice of the Mining Department” comprises numerous 
samples of lava from the dome with fine-crystalline to coarse-grained 
homoeogeneous xenoliths, which will be described lower down. 
The lava of the dome. 
All the rocks examined are brownish-red, porous hypersthene- 
augiteandesites with phenocrysts of zonary plagioclases, among which 
frequently narrow basic and more acid zones occur alternately, so 
that the marginal zone, even in the case of markedly zonary struc- 
ture, is often only little more acid than the central part; also the 
succeeding zones differ but little as to basicity. 
Carlsbad twins occur; in sections of the symmetrical zone we 
determined that, on an average, the composition of the plagioclases 
is like that of bytownite Ab,, Ah,,. Inclusions, among which some 
of ore and of a glassy substance are generally few in number; a 
zonary arrangement, the inclusions being limited to certain zones, 
is sometimes met with. The hypersthene- and augite phenocrysts 
1) R. D. M. VERBEEK et R. FENNEMA. Geologische Beschrijving van Java 
en Madoera. II. 
*) B. G. Escuer. 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. Mijnb. Gen. II. Oct. 1919. 
5) H. A. Brouwer. Crystallisation and Resorption in the magma of the 
volcano Ruang. (Sangi Island). Proc. Kon. Akad. v. Wetensch. Vol. XXIII, 
p. 561. 
