Geology. — “Crystallization and Resorption in the Magma of the 
Volcano Ruang. (Sangi Islands)” By Prof. H. A. Brouwer. 
(Communicated by Prof. G. A. F. MoreNGRAAFF). 
(Communicated at the meeting of January 31, 1920). 
The solid lava from the highest peaks of Ruang, representing the 
oldest visible voleanie products of the island, display microscopically 
a great resemblance to the lava and the dome of the eruption of 
1904, and to the products of the latest eruption '),*. They are 
all hypersthene augite andesites. The extensive rockmaterial which 
was collected by me in 1915 along the slopes of the volcano and 
which may originate from different eruptions, confirms this: nearly 
all the rocks, which were examined microscopically, are also hyper- 
sthene augite andesites; among them occur only few hypersthene 
augite amphibole andesites and a single olivine-bearing rock, viz 
an augite hypersthene amphibole olivine basalt. 
Amphibole and olivine thus appear to belong to the rare minera- 
logical constituents of the magma, which has risen to the surface, 
but the numerous xenoliths, encountered in the ejected products, 
enable us to judge of the crystallization products of the magma at 
greater depth. Especially the homoeogeneous xenoliths *) are very 
numerous. They are not merely mineralogical curiosities, but they 
also indicate what minerals at greater depths of the magma can 
crystallize and they fill up the gaps between the data that are 
obtainable only through the study of the effusive rocks. 
We subjoin a short description of the volcanic rocks of Ruang, 
together with the xenoliths found in them: 
I. Oldest volcanic products. 
Hypersthene augite andesites from the highest peaks of the island 
with phenocrysts of strongly zonary plagioclase, of hypersthene, augite 
1) M. Kopersere, Verslag van een onderzoek naar de uitbarstingen in 1904 op 
het vulkaaneiland Roeang bij Tangoelandang (Sangi- en Talaoet-eilanden). Jaarb. 
Mijnwezen 1909. Wet. Ged. p. 207 e. v. 
3) H. A. Brouwer, Het vulkaaneiland Roeang (Sangi-eilanden) na de eruptie 
van 1914. Tijdschr. Kon. Ned. Aardr. Gen. 1915. 
3) A. Lacrorx, Les enclaves des roches volcaniques, 
