THE ORIGIN OF IGNEOUS ROCKS. 
113 
One objection to the theory that a single magma has sepa¬ 
rated in this way into two or more of very different degrees 
of acidity is the fact that the low percentage of silica in basalt 
is not confined to the feldspar and augite, but is also in the 
base, while the high percentage in rhyolite is in the feldspar 
and still more in the base. Hence the segregation must have 
affected the base even more than the crystals. 
He does not think that the acid or even the intermediate 
varieties can have been primordial masses and have re¬ 
mained liquid from the original earthmass to the time of 
eruption. The primordial magma ought, to possess a con¬ 
stitution similar to basalt. “And, in general, our inference 
from the nature and constitution of the volcanic rocks, from 
their great variety, from the localization of eruptive phe¬ 
nomena, from the intermittent character of volcanic action, 
from the independence of the several vents, is that the lavas 
do not emanate from an earth nucleus wholly liquid, nor 
from great subterranean reservoirs still left in a liquid con¬ 
dition ‘ from the foundations of the world,’ but from the 
secondary fusion of rocks, a part of which may have formed 
the primitive crust, while the remaining part consisted of 
deeply buried and metamorphosed sedimentary strata. We 
must at least admit that the source of lavas is among segre¬ 
gated masses of heterogeneous materials .” 
With regard to the dynamical cause of volcanic eruptions 
he observes that “ there is one general assumption which 
satisfies all the main requisites of volcanism. It is this: 
Volcanic phenomena are brought about by a local increase of 
temperature within certain subterranean horizons .” But this 
does not solve the-problem, since it throws us back on the 
question, What produced the increase of temperature ?—a 
question which he confesses he has been utterly unable to 
answer. There is an alternative assumption, namely, 
“ Deeply seated rocks in regions of high temperature un¬ 
dergo changes, one result of which is to lower their melting 
point.” 
“A relief of pressure is one conceivable mode. Probably 
18—Bull. Phil. Soc., Wash., Vol. 12. 
