1240 
has crystallized as a glass-bearing mass with microlites, just as the 
groundmass of the enclosing rocks. 
That in many xenoliths the amphiboles exhibit a resorption-rim 
where they border on the glass-bearing mass alluded to, points out 
that the amphibole remained stable down to the moment of the 
eruption of the lava dome. After this the pressure in the lava and 
the xenoliths decreased rapidly, which made the amphibole instable 
and augite could crystallize during the time when the temperature 
of the residual magma fell, and a complete solidification had not 
yet been effected. This interval may have been longer or shorter 
for different portions of the dome, hence the resorption in amphi- 
boles of different xenoliths was varying. Already before we accounted 
for the mineralogical differences between xenoliths and dome-lava - 
of the Ruan!) we have assumed that during the eruption of the 
voleano the outpouring magma enclosed fragments of its completely 
or partly solidified dioritie crust. The same applies to dome and 
xenoliths of the Galunggung. The occurrence of pyroxene-poor and 
pyroxene-rich ampbibole-bearing xenoliths may be accounted for by 
the assumption that they originate from zones at various depths in 
this crust. It depends on the difference of pressure and temperature 
of these zones whether amphibole only or first pyroxene and later, 
on further cooling, amphibole has crystallized. *) 
In that case the pyroxene-bearing xenoliths originate from deeper 
zones according as they are richer in pyroxene, whereas at a greater 
depth with a higher temperature much less crystalline components 
and only pyroxenes occur in the outpouring magma, which does 
not contain amphibole as phenocrysts. 
The amphibole-free xenoliths with different structures described 
above, may have crystallized already before the eruption at a great 
depth, so above temperatures, at which the amphibole is stable, 
while in that case complete crystallization has taken place after 
the eruption had commenced, when the amphibole was not stable 
either, in connection with the suddenly modified pressure and tem- 
perature relations. Maybe some of these xenoliths have crystallized 
at a pressure lower than that of the stage of stability of the 
amphibole and may therefore belong to parts of the magma that 
have cooled down more slowly, that could crystallize more com- 
pletely along the walls of the vent and were only then carried 
along by the outpouring magma. 
') HA BROUWER, Crystallizations etc. loc. cit. p. 665. 
3) F. Becker. Gesteine des Columbretes. Anhang. Tscherm. Min. u. Petrogr. 
Mitt. XVI. 1897. blz. 327 e.v. 
