QUARTZ-FELDSPAR SANDS. 873 



some instances important supplementary sources of material. Quartz and 

 acid feldspars have very nearly the same specific gravity, and unless such 

 material be gradually contributed to the water, so that it can be handled by 

 strong waves and currents for a long time before final deposition, quartz 

 and acid feldspar will not be separated. The basic feldspar is more apt to 

 be separated. These conditions for imperfect separation especially prevail 

 in bays and gulfs where the full power of the ocean is not felt. The only 

 other important constituent of ordinary granite and of many of the acid 

 schists and gneisses, beside quartz and feldspar, is mica. This mineral, 

 because in flakes, is carried farther than the quartz and feldspar, and thus is 

 easily and rapidly separated from them and transported to deeper water. 

 Consequently, in the comparatively shallow waters along the shore many 

 nearly pure quartz-feldspar sands may be built up. 



One of the best illustrations of the deposition of quartz-feldspar sands 

 at the present time is that in ~the Gulf of California, described by McGee." 

 The rocks of this region consist largely of granites and granitoid schists and 

 gneisses, although mingled with them are various igneous rocks and tuffs. 

 The region is one of great aridity and rapidly varying temperature. The 

 small amount of rainfall is concentrated in thunder storms, in some cases of 

 exceptional violence. The result is that the rocks which are disintegrated 

 but not decomposed are transported by the process which McGee has 

 called sheet-flood erosion. (See Chapter VI, p. 497.) The undecomposed 

 material is piled up in the ravines of the mountains or is transported to the 

 gulf by storms of exceptional violence. The waves of the gulf are at work 

 upon such disintegrated material and the salients of the solid rock. Thus 

 a large amount of material is furnished by the waves and streams, to be 

 distributed by the shore currents and undertow. The final result is that 

 the quartz and feldspar, with some mica, are deposited as a quartz-feldspar- 

 sand formation. McGee notes that where there are prominent salients the 

 quartz-feldspar sand is coarsest and cleanest, and that where there are 

 reentrants there is mingled with the coarse quartz and feldspar a considerable 

 proportion of finely comminuted materials.' 



Doubtless this case of the formation of quartz-feldspar sands in the 

 Gulf of California points to the conditions under which similar sands were 



"McGee, W J, The formation of arkoae: Science, new ser., vol. 4, 1896, pp. 962-963. 

 b McGee, cit, pp. 962-963. 



