CRYSTALLIZATION IN IGNEOUS ROCKS 465 
porphyry and granite-porphyry to a normal granite, does not 
bear out this conclusion, and is explicable only on the basis of the 
order here deduced, and a gradual change in rate of cooling. Indeed, 
the resorption phenomena exhibited are best explained as due to 
the sudden change of conditions experienced by a rhyolite, especially 
the relief of pressure. The specific volume of quartz is 0.377 and 
of “quartz glass” 0.452 at 25°. The ratio of the values of these 
quantities at higher temperature is probably of the same order. 
The great difference between these figures indicates the strong 
tendency that the sudden relief of pressure would have in the 
direction of re-solution of quartz in a magma from which it had 
separated. 
If the crystallization of other rocks as determined by taking 
the evidence of plutonic and effusive types regarded as their 
quenched equivalents is represented in diagram, we obtain, 
F 
Syenite. 2 
pP—_—_— 
A = 
a— 
Q 
Quartz Diovute. c 
[7 
P 
A— 
Cc 
Diorute. P 
F— 
j—— 
A— 
P 
Gabbyo. c 
F ————— 
Fic. 6 Co 
