METAMORPHISM BY IGNEOUS AGENCIES. 711 



XES UL TANT METAMORPHISM. 



It is natural to expect that the rocks surrounding great batholiths of 

 igneous rocks, and those intimately associated with the abundant and 

 numerous branch batholiths and dikes, would be profoundly modified, and 

 such is the fact. Conditions for profound and far-reaching modification are 

 perhaps most favorable in connection with batholithic injection and conjoint 

 orogenic movements, and to this fact is probably due the classification of 

 certain kinds of metamorphism as contact metamorphism. But it must be 

 remembered that, with the one exception, the forces and agents at work are 

 essentially the same as those in other parts of. the zone of anamorphism, 

 the difference being, merely, that under the conditions of batholithic 

 intrusion these forces and agents are particularly potent. During an 

 intrusion the orogenic forces and the thrust transmitted by the magma 

 both act upon the rocks, and thus the mechanical processes are at work. 

 The temperature is abnormally high; hence the solutions are especially 

 potent. To some extent, as pointed out on page 662, the magma itself 

 may furnish water, and thus the surrounding rocks contain more than an 

 average amount of water; and this is the most important agent in recrys- 

 tallization. Therefore about great batholiths are usually zones of profound 

 metamorphism. 



FACTORS CONTROLLING METAMORPHISM. 



The factors of alteration to be especially considered in this connection 

 are the size of the intrusive masses, the temperature, the amount of water 

 present, and the composition of the intrusive and intruded rocks. 



SIZE OF INTRUSIVE MASSES. 



The size of the intrusive masses is of the first importance in the 

 amount of metamorphism. The smaller of the deep-seated irregular masses 

 are ordinarily called stocks; those of intermediate size, bosses; and those 

 of the largest size, batholiths. We know of no downward limit for any of 

 these'. Oftentimes they undoubtedly increase very rapidly in magnitude 

 with depth, although this fact can not be directly observed. Erosion may 

 have extended only deep enough to expose a very small area of a bath- 

 olith, and yet for long- distances about that place the subtuberant mass may 

 be only a short distance below the surface, and the rocks exposed therefore 

 be well within the limit of its influence. This must be a common case, for it 



