874 A TREATISE ON METAMORPHISM. 



deposited in the past, although other combinations of conditions may have 

 produced similar results. For instance, where the sea has rapidly trans- 

 gressed over a region of granitic rocks the material may be broken down 

 by wave action and incompletely assorted, thus producing quartz-feldspar 

 sands. Or great deposits formed in an interior basin may be overridden. 

 Thus if by subsidence the sea should advance over the Great Basin of 

 western United States (see Chapter YI, p. 559) at various places stratified 

 feldspathic sands similar in character to those of the Gulf of California 

 would be buried, since in this region there are various granitic and acid 

 schistic, gneissic, and granitic mountain ranges about which the fieldspar 

 and quartz debris of the mountains is being built up as stratified rocks by 

 the streams and ephemeral lakes and by sheet flood erosion. But from 

 such deposits neither mica nor any other mineral is more than imperfectly 

 separated. Therefore, unless the original rocks were enormoitsly rich in 

 quartz and feldspar, such deposits are likely to contain with the quartz- 

 feldspar sands considerable quantities of ferromagnesian sands, considered 

 on pages 877-879. 



ARKOSE. 



Arkose is cemented quartz-feldspar sands. The arkoses are formed in 

 the belt of cementation. The cementing material is largely quartz, just as 

 with the quartz sands. Other cementing materials are calcite, ferrite, etc., 

 and in some cases the cementing material is largely feldspar. This is well 

 illustrated by the arkoses of the Keweenawan of the Lake Superior region. 

 The rocks from which these arkoses were derived were the volcanic rocks 

 of the Lower Keweenawan. In the Keweenawan arkoses the cementing' 

 material is mainly quartz, but this is occasionally supplemented by feldspar, 

 and in some instances where the arkose is a nearly pure feldspar rock the 

 feldspar interstitial material is the main cement. This feldspar is largely 

 added to the feldspar grains in optical continuity, precisely as is the quartz 

 to the quartz grains, thus enlarging them. Where the feldspars are twinned 

 plagioclases, the enlargements are twinned in similar fashion." (See fig. 14, 

 p. 626.) These arkoses give one of the best illustrations of the capacity of 

 mineral particles to abstract from the solutions material like themselves and 

 add it to themselves. (See pp. 121-123.) 



"Irving, R. D., and Van Hise, C. R., On secondary enlargements of mineral fragments: Bull. 

 U. S. Geol. Survey, No. 8, 1884, p. 46. 



