But here again the numerous resorption-phenomena demonstrate the 
instability of this mineral under circumstances different from those 
which prevailed during crystallization. The phenomenon may be 
compared with the corrosion-phenomena of rhombic pyroxene in 
xenoliths of basalts.') In similar rocks, which are more basie and 
richer in lime than the effusiva of Ruang, phenocrysts of rhombic 
pyroxene occur very rarely, nevertheless this mineral is found in 
xenoliths mostly in a corroded condition. The formation of ortho- 
silicate instead of metasilicate is, under otherwise similar conditions, 
dependent on the quantity of available silica, which can combine 
with Mg and Fe, but many instances are known in which olivine 
is crystallized in magmas, which contain enough silica to give 
origin to metasilicate (SiO,-rich basalts). The co-incidence of pyro- 
genetic quartz and olivine in the same rock has been explained 
by the action of the water-vapour present in the magma’) which 
has impeded the formation of the metasilicate. 
The xenoliths of various mineralogical composition and of different 
structures point to erystallizations which have taken place under 
various circumstances and very likely at widely different depths in 
the magma. The various types are connected by intermediate struc- 
tures. We can account for the great abundance of amphibole-bearing 
xenoliths and the striking contrast of the absence of amphibole- 
phenocrysts in the lava by assuming that the magma beneath Ruang 
was in its upper parts, before the commencement of the eruption, 
under pressure- and temperature-relations, in which first pyroxene 
and later on at a subsequent cooling amphibole could crystallize, while 
at greater depths the field of crystallization of the amphibole was 
not reached. At the commencement of the eruption the upper portions 
of the magma were crystaliized completely or for the greater part, 
while the magma with fewer and different crystalline constituents 
and with greater liquidity lay at a greater depth, which magma 
was effused at an eruption of the volcano as a hypersthene augite 
andesite and presented the fragments of its dioritie crust which had 
been solidified completely or partially, as xenoliths. 
1) A. Lacrorx, Les enclaves des roches volcaniques, p. 491. 
3) J. P. Ippines, Igneous Rocks. Vol. I. 1909, p. 142. 
