126 
IDDINGS. 
highly silicious varieties. The rocks grade into one an¬ 
other, petrographically and chemically, and in some places 
in the field form continuous bodies of variable composition; 
in others they constitute definite bodies of different char¬ 
acters. 
“ Taking all the facts into consideration,” the writers re¬ 
mark, “there seems no escape from the conclusion that we 
have in this area the record of a series of events connected 
with the consolidation of a vast subterranean Reservoir of 
molten rock. Having regard to the whole mass, it seems 
probable that the process was a continuous one; but if we 
consider only certain limited portions, it was unquestionably 
discontinuous.” * * * “ The order of succession of the 
rocks is from basic to the most silicious. The order in which 
the essential minerals of these rocks commenced to crystallize 
from the magmas is said to be the same in all cases and to 
begin with the most basic and specifically heaviest and to 
end with the most silicious and specifically lightest. The 
variation in chemical composition is also considered and 
plotted in a diagram representing the molecular proportions. 
In suggesting a cause for the observed relationship between 
the rocks of this group the idea is expressed that “ the area 
represents a vast subterranean reservoir, which has become 
differentiated during the process of consolidation,” and that 
the separation was a consequence of the formation of the 
minerals in the order just given. 
“ Other causes, such as Soret’s principle, or the subsidence 
of the first formed crystals by gravity, may also operate and 
assist in producing heterogeneity in the original magma.” 
In attempting to apply this hypothesis to the problem of 
volcanic rocks it is concluded that the basic rocks should 
precede the acid rocks; but it is noted that such a rule is by 
no means without exceptions. “ The exceptions may be due 
to the existence of two or more reservoirs, or to the accession 
of heat or fresh material during the process of consolidation.” 
It is also suggested that “ the consolidation of a plutonic 
magma may give rise to rocks of which there are no volcanic 
