METAMORPHOSED ARKOSE. 875 



The process of cementation may be partial, as with the sandstones, or 

 may continue until the material has practically rilled all the interspaces, as 

 with quartzites, when the rock becomes completely indurated arkose. 

 Where cementation is very partial, the rock fractures around the grains 

 rather than across them, just as in a sandstone. Where cementation is 

 complete or nearly so, the fractures are across the grains rather than 

 around them, just as in quartzite, to which the rock is then analogous. 



During cementation the feldspars in the arkoses may undergo meta- 

 somatic changes. The dominant feldspars are orthoclase, microcline, and 

 the acid plagioclases. Any of the alteration products of these minerals 

 characteristic of the belt of cementation may form, but the most abundant 

 products of their alteration are quartz, chlorite, the zeolites, and the 

 epidotes. 



Where by deep burying the arkoses pass into the zone of anamorphism, 

 the original feldspars or their alteration products are likely to recrystallize, 

 producing heavy anhydrous minerals. Under such circumstances the acid 

 feldspars are most likely to pass into quartz and mica. The alteration 

 products of the feldspars produced in the belt of cementation may 

 reproduce feldspar or may unite to form quartz and mica. In proportion 

 as the quartz-feldspar sands were impure, the heavy minerals are likely to 

 form. So long as the conditions are mass-static the original textures and 

 structures of the rock are likely to be preserved; but if in the deep-seated 

 zone the conditions are mass-mechanical, schist-arkoses or gneiss-arkoses, 

 considered under the next heading, are likely to form. 



The induration of the arkoses is believed to be mainly accomplished 

 while the deposits are land areas. The reasons for this belief are the same 

 as in the case of the sandstones. (See pp. 866-867.) 



SCHIST-ARKOSE AND GNEISS-AREOSE, OR AREOSE-SCHIST AND ARKOSE-GNEISS. 



After the arkoses are formed and have become buried deep enough to be 

 in the zone of anamorphism, mass-mechanical action may take place. In 

 consequence of this a schistose or gneissose structure is developed, and the 

 rocks become schist-arkose aiid gneiss- arkose, or arkose-schist and arkose- 

 gneiss. There are, of course, all gradations between the rocks here con- 

 sidered and the arkoses. Where the mechanical action is rather weak, 

 a schistose structure is not marked. Where mass-mechanical action is 



