THE STRENGTH OF THE EARTH'S CRUST 515 



extreme deformation is by faults and folds. With increasing depth 

 and strength the joints become less abundant and faults pass into 

 flexures. The passage of fractures into flexures implies the begin- 

 nings of massive flow. Where magmatic heat or emanations are 

 not present the mode of mashing is presumably more especially 

 by granulation. With still greater depth the yielding becomes 

 more uniformly distributed throughout the rock mass. Both 

 because of this pervasiveness of mashing and the great strength 

 of this zone, deformation here requires the most force and absorbs 

 the most energy of any part of the lithosphere. At greater depths 

 the rock is more compressed, and is still more rigid than above, 

 but the temperature here approaches fusion; recrystallization 

 readily takes place, the strain which can be elastically carried is in 

 consequence low, and the lithosphere passes gradually into the 

 asthenosphere. Where, however, magmas rise through the crust 

 they carry with them the environment of the asthenosphere; the 

 lithosphere becomes locally abnormally heated and saturated 

 with magmatic emanations. Recrystallization goes forward readily 

 and the zone of weakness penetrates upward even to the zone of 

 fracture. Thus in the injected and crystalUzed roofs of ancient 

 batholiths, laid bare by profound erosion, we may perceive the 

 nearest approach to dynamic conditions which prevail in depths 

 forever hidden. 



