308 Tlie A7nerican Geologist. May, i9oo 
where the phenocrysts are large and distinct and quite cer- 
tainly intratelluric in origin. But in those far more common 
and important cases where the phenocrysts of lavas are small, 
numerous and inconspicuous, increasing viscosity and con- 
sequent over-cooling seem to be demanded, as much as for 
intrusive masses and the contact zones of batholiths or pluto- 
nic masses. 
The porphyritic texture of plutonic rocks is not, however, 
wholly confined to the more superficial portions or contact 
zones of batholiths. In proof of this it is sui^cient to cite the 
great bodies of coarsely prophyritic granite, with feldspar phen- 
ocrysts from one to several inches in diameter, which are a 
conspicuous feature in the geology of Xew^ England and other 
regions. It may be that in some cases such bodies represent 
the superficial portions of the batholiths to which they belong, 
or that they are intrusive, the phenocrysts both antedating 
and post-dating the intrusion, having continued to grow after 
the intrusion. But surely they belong to an entirely distinct 
category from the quartz porphyry and fine-grained granite 
porphyry of the normal contact zones ; and of one thing we 
may be very confident — these coarse, granite porphyries have 
in every instance been formed at a depth so great as to make 
it in the highest degree improbable that either sudden cooling 
or over-cooling of the magma will explain the gigantic phen- 
ocrysts, or rather the striking contrast between them and the 
groundmass. 
What I am inclined to regard as a very normal occurrence 
of this type of granite attracted my attention in the high Si- 
erras, during the excursion, previously mentioned, frorr 
Fresno, California, to the head w^aters of the San Joaquin 
river. The canyon of this stream gives a nearly complete sec- 
tion of the Sierras, cutting to the very heart of this great, tilted 
orographic block of granitic rocks. The original sedimentary 
cover of this part of the batholith is represented now only by 
occasional limited patches of metamorphic slate standing verti- 
cally in the granite on the westward-sloping upper surface of 
the orographic block as represented by the crests of the ridges 
separating the tributaries of the San Joaquin. Between these 
points and the bottom of the main canyon, the difference of 
level ranging from 4.000 to 5.000 feet, several very instructive 
