880 A TREATISE ON METAMOEPHISM. 



During cementation of the grits, metasomatism in the feldspars, ferro- 

 magnesian minerals, and constituents other than quartz is taking place. 

 This process is precisely the same as in the metasomatism of the gray- 

 wackes, next taken up; and since in these rocks the process is of greater 

 consequence, it will not be here considered further than to remark that the 

 expansion reactions which take place contribute material to the solutions 

 for cementation. 



It is believed that the chang-e of the ferromagnesian sands to grits is 

 mainly accomplished on the land areas. The argument is the same as with 

 the graywackes. Since it can better be given after these rocks have been 

 considered, the statement on this point is deferred. (See p. 882.) 



Grits of Carboniferous age are rather extensive formations in various 

 parts of the world. This is natural, since coal is known to have formed in 

 lagoons and partly inclosed seas, and it has already been noted that such 

 places, where the sorting power of the waves is small, are favorable ones 

 for the building up of the ferromagnesian sands. But the grits are not b}^ 

 any means confined to the Carboniferous system; they occur extensively 

 in the Cretaceous, Tertiary, and other systems. 



GRAYTVACKE. 



Graywackes are produced from grits in the belt of cementation. 

 Where the cementation is advanced so far that the rock when fractured 

 breaks across the original grains rather than around them the rock is a 

 graywacke. (PI. X, A.) Craywacke, therefore, occupies the same posi- 

 tion in the family under consideration that quartzite and completely indu- 

 rated arkose occupy in the quartz-sand and quaxtz-feldspar-sand families. 

 Nolhing further need be said of the process of cementation, since it is mainly 

 the completion of the work which cements a ferromagnesian sand into a 

 grit. While the process of cementation is going on metasomatic changes 

 may also be occurring in the body of the rock. (See Chapter VII, pp. 

 640-646.) The clastic particles may alter in many ways. To describe 

 them in detail would be to give a treatment of the alteration of minerals. 

 The final result of the alteration is a consolidated rock which may have a 

 very great variety of mineral constituents. One of the most characteristic 

 features of the graywackes is, however (as indicated by the .name), their 

 gray color. While the new minerals may have something of a tendency 



