304 The American Geologist. May, 1900 
together. A rapid solidification of the groundmass or mag- 
ma residuum is thus seen to be probable. The su])erficial 
cooling of tlie batholith probably went on slowly at first, and 
more rapidly afterward, which would be a condition much 
more favorable, though not strictly essential, to the develop- 
ment of tlie porphyritic texture in situ. The fact that in the 
subsequent intrusive and effusive outflows of magma from the 
interior of the batholith the phenocrysts are few and small is 
certainly strongly against the view that the phenocrysts of the 
quartz porphyry were formed at a considerable depth in the 
batholith. 
The development of the condition where the rate of solid- 
ification overtaxes molecular flow would, of course, be ex- 
tremely gradual ; Ijut the resulting change in the mode of crys- 
tallization would, I firmly believe, be abrupt, although not 
necessarily simultaneous for any considerable body of magma. 
I have compared the magma at this stage to an over-cooled 
liquid or supersaturated solution ; and I see no escape from 
the conclusion that in accordance with well-established 
physical laws, a condition of unstable equilibrium must result, 
when the magma will be actually liquid but potentially solid; 
so that diffused crystallization when once started in any 
part of the magma, will proceed rapidly. As I regard the 
matter, any rate of cooling not extremely slow would suffice, 
the change occurring at a higher temperature with a rapid 
rate and a lower temperature with a slower rate. If the rate 
of cooling is slow enough there is no change and allotriomor- 
phic crystallization is the result; but if it is more rapid, the 
overtaxing of molecular flow and development of porphyritic 
texture at a temperature proportionate to the rate appear to 
be inevitable. We are thus able, consistently with the un- 
doubtedly slow and gradual changes of plutonic conditions, 
to explain an abrupt change in the crystallizing process. The 
old theory completely breaks down at this point; that is, it ap- 
peals to a sharp change of temperature, pressure or hydra- 
tion, but utterly fails to explain how either is possible in a stag- 
nant or sedentary magma at a great depth in the earth. 
One pertinent and inevitable criticism of the new view 
may be anticipated at this point. To the statement that any 
rate of cooling not extremelv slow would suffice for the de- 
