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affected large areas of the pre- Cambrian rocks of North America." 

 The author recognizes pressure, heat and chemical affinity as the 

 prominent forces producing metamorphism. When pressure is resisted 

 by rigidity, i. e., produces only molecular motion, the result is static 

 metamorphism ; when pressure passes into mass motion the result is 

 dynamic metamorphism. The chief processes considered are consoli- 

 dation, welding, cementation, injection, metasomatism, and mashing. 



In discussing cementation and injection the author takes up the 

 origin of pegmatites and says : 



" It seems to me that to adequately explain all the facts of pegmat- 

 ization described in various regions of the world, we must conclude 

 that all three processes have been at work — in some cases igneous 

 injection, in some cases aqueo-igneous action, and in other cases pure- 

 water cementation, and in still others combinations of two or all of 

 these processes. It is further believed that there may be no sharp 

 separation, but, on the contrary, all gradations between the three — 

 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 solu- 

 tions, true injection. Pegmatization may comprise these and the 

 intermediate processes." 



Metasomatism is described with reference to (1) "alteration of 

 original mineral particles, (2) minerals produced by alterations, and 

 (3) texture of rocks produced." The brief statement of facts familiar 

 to petrographers suggests also broader aspects of comparative 

 petrology. 



" Mashing is a term here introduced with a specific meaning. It 

 is used to designate by a single word that complex process which 

 involves shortening without change of volume, or simple shearing, 

 or a combination of the two. Mashing implies all the other special 

 processes of metamorphism." 



Having thus considered in general the processes of metamorphism, 

 Van Hise classifies metamorphic sedimentary rocks according to those 

 processes which have been particularly effective in the development of 

 each class. The enumeration of rock types in relation to conditions 

 affecting their development is particularly valuable. 



