46 The American Geologist. 
July, 190S> 
primal magma of the batholite, we have now considered all 
of the sedentary or truly plutonic rocks of the batholite. 
These have been described in the order of superposition, 
which is, of course, the inverse order of age, since the re- 
frigeration of the batholite must have progressed from the 
periphery downward or centripetally. That the differenti- 
ation of the normal granite and the contact zone, and the 
further differentiation of the fine granite and quartz por- 
phyry of the contact zone, are not wholly textural is, as 
noted by Dr. Bascom, clearly shown by analyses, according 
to which the normal granite is the most basic and the fine 
granite the most acid, while the quartz porphyry is inter- 
mediate in composition, although not so in position. By 
way of explanation of this chemical relation, Dr. Bascom 
has suggested that specific gravity and convective currents 
may have been factors in producing a somewhat more acid 
peripheral zone to the batholite, while the outer or quartz 
porphyry border to this zone, following the general law 
of the order of crystallization (virtually fractional crystalli- 
zation), by its earlier crystallization left the inner portion 
-of the zone or fine granite more acid than either the quartz 
porphyry or normal granite. To this explanation may, 
perhaps, be added the influence of hydration. It appears 
reasonable to suppose that the primal magma of the batho- 
lite, formed under a thick and, necessarily, a hydrated sedi- 
mentary cover, and due in part to the absorption of large 
volumes of this cover, would naturally be more highly 
nydrated in its superficial than in its deep-seated portions; 
and since the characteristic elements of an acid magma, in- 
cluding silica and the alkalies, have a stronger affinity for 
water than have the lime, magnesia and iron oxide charac- 
teristic of basic magmas, we have here a cause tending to 
T<eep, if not to make, the batholite superficially acid. 
That, in comparison with the granitic rocks of other 
districts, this part of the batholite " was formed under a 
moderate depth of cover, is believed to be indicated by the 
relatively slight amount of chemical differentiation, by the 
absence of an original micaceous constituent, and especially 
1>y the almost entire absence of a pegmatitic phase in* the 
normal granite. The absence of marked differentiation ex- 
