758 A TREATISE ON METAMOKPHISM. 



dimensional arrangement, these give no basis to estimate the amount of 

 shortening of the rock. 



The case of flattened original fragments is very different. In so far 

 as the resultant material or flattened ovules can be identified with original 

 forms, as, for instance, where conglomerate pebbles and fossils are deformed, 

 these distorted objects give a means of determining the amount of the mass 

 shortening. 



In proportion as the conditions are unfavorable for recrystallization — 

 that is, as mineral particles are refractory, as they are coarse grained, as the 

 deformation is rapid, as the depth is small, as the temperature is low, as the 

 water content is small — granulation occurs instead of recrystallization. This 

 is sufficiently evident, without amplification, from the discussion already 

 given. (See pp. 737-748.) 



In the imperfect slates, schists, and gneisses, which are widespread 

 rocks, adjustment during deformation is accomplished in part by the 

 process of recrystallization just described, but also in part by the mechan- 

 ical processes. Of these, granulation with differential movement of the 

 granules and welding of the granules is of first importance. Strain within 

 the elastic limit is of very considerable moment. The deformation of the 

 mineral particles by movement along gliding planes may be of occasional 

 importance, especially in certain classes of rocks, such as marbles. 



The cataclastic rocks do not usually show slaty or schistose structures. 

 In these rocks the process of flowage is mainly mechanical. It is chiefly 

 accomplished by the multitudinous fracturing of the solid particles, hj 

 differential movements between the minute particles, and by the welding 

 of the differentially moved bodies. With this process as a main one, there 

 is sure to be marked strain within the elastic limit in all the particles, both 

 original and secondary; and thus this is a rather important factor in the 

 process. Finally, gliding is more important than in the case of recrystalli- 

 zation-flowage. But it may perhaps be doubted whether even in the typical 

 granulated rocks the process of flowage is wholly mechanical. In all rocks 

 there is a small amount of water present, and even in the instances of 

 apparently perfect granulation probably solution and deposition, or recrys- 

 tallization, has taken place to some extent. Where deformation in the zone 

 of anamorphism is mainly accomplished by granulation and differential 

 movement of the particles, the process may be called granulation-flowage, 

 or mechanical flowage. 



