METAMORPHISM 349 



been discussed, it remains to be shown why metamorphic rocks are 

 usually cleavable. 



1. Crystallization. — A study of a mica or hornblende schist 

 shows that hornblende and mica are responsible for the best rock 

 cleavage. A microscopic examination of these rocks and of the 

 sedimentary rocks from which they were derived shows that horn- 

 blende and mka were built up chiefly by subsequent recrystalliza- 

 tion from substances already in the sedimentary rocks and did not 

 exist in them in their final form. This fact is shown by a general 

 lack of fractures in the minerals of the cleavable rock, such as would 

 have been developed had the rock been formed simply by crushing 

 and by the rotation of the mineral constituents to parallel positions. 

 Moreover, most of the mineral particles of cleavable rocks are larger 

 than those of the same rock before the latter was metamorphosed. 

 The gradation of shale to phyllite (a metamorphic shale) means an 

 increase in the size of the grains. The parallel arrangement of the 

 mineral constituents of a metamorphic rock is thus seen to be the 

 result of crystallization, and the rotation of the original particles 

 to a parallel position, of minor importance. 



2. Granulation. — Recrystallization and rotation are not the 

 only processes instrumental in the production of easy splitting or 

 cleavage in metamorphic rocks. In the early stages of the process 

 the larger brittle particles are broken into small fragments or are 

 granulated and elongated, and at the same time recrystallization 

 builds up new minerals from the broken particles. 1 It is probable 

 that granulation aids crystallization in that it grinds the particles 

 into small pieces which then present a greater surface upon which 

 the chemical process may act. 



Relation of Cleavage to Pressure. — The proof that the planes 

 of easy splitting of metamorphic rocks are at right angles to the 

 pressure, or in other words parallel to the rock elongation, is seen (1) 

 in the distortion of the pebbles of conglomerates, (2) in the distortion 

 of fossils, the lengthening being in the plane of the cleavage, and (3) 

 when rock is intruded by igneous masses which exert a great pressure 

 on the walls, in the cleavage which is developed parallel to the walls. 



From Igneous, through Sedimentary, to Metamorphic Rocks. — 



The history of a metamorphic rock, formed by the recrystallization 



of sedimentary rocks, may be briefly summarized. If a great mass 



of granite is exposed to the weather, it begins to decay; its feldspar 



1 Leith, C. K., — Structural Geology, 191 3. 



