DYNAMIC METAMORPHISM 769 



took place. In such a case, the effect of movement is seen on the 

 mineral. 



In general, when extensive dynamic disturbances take place, 

 resulting in crushing and mashing of the rocks, new minerals and 

 new structures are developed. The former are numerous ; among 

 the latter are cleavage, fissility, and schistosity. 



Different types of rocks suffer different alterations under the 

 influence of the mass-mechanical motion characteristic of symphrat- 

 tism. A few of these alteration products may be mentioned, but 

 the student is referred to special treatises on the subject for more 

 detailed information. 



When coarse, clastic rocks, or rudytes, are subjected to dynamic 

 metamorphism, there will generally result a recrystallization and 

 granulation, and the development of schistose structure. This is 

 especially the case in the matrix in which, owing to its great range 

 in composition, a large variety of minerals may be developed. A 

 schist conglomerate or conglomerate schist is produced, generally 

 with the pebbles flattened and elongated and more or less granulated 

 and recrystallized. With extreme movement, the pebbles may be 

 flattened into laminae or changed into a variety of minerals accord- 

 ing to their original composition. A quartz pebble may thus be 

 drawn out into a lamina of granulated quartz, often only as thin as 

 cardboard or even as paper. Granite pebbles may be transformed 

 into a micaceous lamina with quartz and feldspar grains. The ma- 

 terial of the original pebble may become more or less commingled 

 with the matrix so that the outline of the pebble disappears and 

 finally all trace of the conglomeratic character is lost, the mass 

 being a schist with laminae of varying composition interspersed. 

 The matrix may become progressively slate, schist and foliated 

 schist, the particles at first winding in and out among the pebbles, 

 but becoming more parallel as the pebbles disappear. 



Impure arenytes and lutytes may suffer changes similar to those 

 of the matrix of rudytes. Clastic gneisses and schists are thus 

 produced, or exogneisses and exoschists, since they are derived from 

 exogenetic rocks. They are distinguished from endogneisses and 

 endoschists, or those produced from endogenetic (chiefly pyrogenic) 

 rocks, by the parallel orientation of their mineral particles, which 

 gives cleavage to the rock. This structure is almost if not quite 

 universal with the schists and gneisses derived by the metamorphism 

 of sedimentary rocks, but is commonly lacking in schists and 

 gneisses produced from igneous rocks. 



The alteration of other sedimentary rocks by dynamic meta- 

 morphism may be briefly reviewed. Thus quartz sandstone changes 



