MASS DYNAMIC ACTION. 311 
critical temperature between those processes, it is probable that about 
5,000 meters, under conditions of slow deformation, is a critical depth 
between the two processes, 
_ However, it must be remembered that the mechanical work of defor- 
mation itself develops heat, which can only escape by conduction or 
convection. Therefore during mountain-making periods this tempera- 
ture may have been attained much nearer the surface than 5,000 meters, 
and consequently recrystallization take place rapidly at depths much 
_ less than this. 
Moreover, the temperature is raised by the presence of intrusive 
igneous rocks. The heat of the intrusives is conveyed to the adjacent 
rocks, both by conduction and by convection through water. Hence 
the presence of igneous rocks is favorable to recrystallization. This is 
so well known that it need not be dwelt upon. The broad zones of 
schists in which recrystallization is complete about the great batholiths, 
with schistosity everywhere parallel to the sides of the intrusives, are so 
numerous and well known in almost every country as to require no 
discussion. Well known American instances are the mica-schists and 
mica-eneisses about the Harney Peak granite of the Black Hills* and 
the zones of schists about the granite batholiths of western Massachu- 
setts. 
The identical character of the schists surrounding batholiths and 
those produced by regional dynamic action is explained by the forego- 
ing pages. The necessary conditions for the production of recrystallized 
schists are movement under sufficient pressure, moderate temperature, 
and presence of water. These conditions are available in the two cases 
mentioned in different ways. In the case of the schists about batho- 
liths the pressure is furnished by orogenic forces, and probably very 
largely by the intrusive itself. The heat is largely furnished by the 
igneous rock. If the intrusives are within the zone of fracture, water 
may readily gain access to the surrounding rocks. 
Pressure and rapidity of deformation.—The less the pressure the 
more likely is the deformation to be accomplished by granulation. The 
greater the pressure the more likely is the deformation to be accom- 
plished by recrystallization. The pressure increases with depth, with 
mechanical action, by igneous intrusions, and possibly by other causes. 
To a certain point, the more rapid the deformation the more likely is 
the adjustment to be by granulation. The limit beyond which this does 
not apply is reached when the mechanical process develops sufficient 
heat, so that the readjustment is by recrystallization rather than by 
*The pre-Cambrian rocks of the Black Hills, by C. R. Van Hise: Bull. Geol. Soe. Am., vol. i, 
1890, pp. 206-210. 
