690 A TREATISE ON METAMORPHISM. 



of uniform size and parallel orientation, and there is a tendency to destroy 

 old structures and to produce a characteristic slaty or schistose structure. 

 In metamorphism under mass-static conditions original textures and struc- 

 tures are usually preserved, although they may be somewhat modified or 

 emphasized by the unequal size and lack of orientation of the newly- 

 developed mineral particles. 



Theory of recrystaiiization. — Where recrystallizatioii occurs in the deep-seated 

 zone the temperature is considerably higher than at the surface, because of 

 the increase of temperature due to depth, because of heat resulting from 

 mechanical action, and in many districts because of heat derived from 

 intrusive igneous rocks. Water occupies all the openings, including those 

 of subcapillary size. Moreover, this water has about the temperature of 

 the adjacent rocks, and is therefore extremely active. Taking the ordinary 

 gradient, the temperature at a depth of 3,000 meters would be 100° C; at 

 6,000 meters, 200° C; and at 9,000 meters, 300° C. At these temperatures 

 the material would ordinarily be water and not steam, for, ignoring the rock 

 pressure, the pressure of the superincumbent column of water is more than 

 sufficient to jDrevent it from passing into the condition of a gas. But in 

 consequence of the heat of mechanical action or of igneous intrusion, or 

 both, the temperature at a given depth may be so high that the water may, 

 at least locally and for short times, be in the form of a gas. During the 

 mass movements of rocks water as liquid makes its way between the rock 

 particles much more readily than under conditions of quiescence. (See 

 p.^664.) Therefore the water, on account of high temperature and com- 

 paratively free movement, is in a most favorable condition for work. 



It has been pointed out (p. 98) that during- orogenic movements the 

 rock pressures vary from place to place and from moment to moment. 

 This results in great variation in the pressure upon the contained water. 

 When the pressure increases, solution takes place; when it decreases, depo- 

 sition occurs. Therefore, in consequence of changing pressure during 

 orogenic movement the conditions are favorable for alternate solution and 

 deposition. Since the pressure continuously varies throughout long periods 

 of orogenic movement, it is probable that this is a factor of very consider- 

 able importance in the recrystaiiization of the rocks. 



Another factor which is of great importance in recrystaiiization is the 

 potentialized energy which exists in mineral particles in consequence of a 



