Van Hise and Hoskins — Pre- Cambrian Geology. 211 



Autoclastic Rocks. — When rocks are folded by strong oro- 

 genic forces, and they are not so heavily loaded as to render 

 them plastic, they are frequently broken into fragments, and 

 '' autoclastic " rocks are produced. The autoclastic rocks which 

 readily show their origin may be called breccias, and those 

 which resemble ordinary conglomerates may be called pseudo- 

 conglomerates. Brittle rocks are the most likely to become 

 autoclastic. Autoclastic rocks are confined to the zone of frac- 

 ture and the zone of combined fracture and flowage. Pseudo- 

 conglomerates sometimes resemble basal conglomerates. Hence 

 criteria are described which discriminate between the two. 



Met amor phism. — Metamorphism is used in its broad sense 

 to cover the alteration of all classes of rocks by all processes. 

 The prominent forces producing metamorphism are pressure, 

 heat, and chemical affinity. Pressure may be resisted by 

 rigidity, or may pass into mass motion. In the former case 

 the conditions are those of static metamorphism, and the 

 motions are molecular. In the latter case the conditions are 

 those of dynamic metamorphism, and the motions are both 

 molecular and mass. The heat may result from dynamic action, 

 be obtained from liquid rock intruded from below, or from the 

 interior of the earth by the normal increase of temperature due 

 to depth. In the chemical changes water is the menstruum 

 through which most of the transformations are made. It car- 

 ries the materials from one place to another, and thus the com- 

 position of great masses of rock may be changed. 



The chief processes of metamorphism considered are con- 

 solidation, welding, cementation, injection, metasomatism, and 

 mashing. Consolidation is produced by the mere pressure of 

 superincumbent strata or by the stress of thrust, so that the 

 mineral particles are brought closer together. If rocks are so 

 deeply buried that the superincumbent weight is beyond the 

 crushing strength, or if they are in the zone of flowage, it is 

 possible that the particles may become welded together. By 

 cementation is meant the binding together of the rock parti- 

 cles by the infiltration of mineral material in solution and its 

 deposition as minerals in the interstices of the rocks. By 

 injection is meant the penetration of a rock by a molten 

 magma. Pegmatization is believed in some cases to be due to 

 cementation, in others to injection, and in others to a combina- 

 tion of the two, that is, it is thought highly probable that 

 under sufficient pressure and at a high temperature there are all 

 gradations between heated waters containing mineral material 

 in solution and a magma containing water in solution. In 

 other words, under proper conditions water and liquid rock are 

 miscible in all proportions. From the water solutions true 

 impregnation or cementation would take place ; from the rock 



