384 Noble — Geology of the Grand Canyon, Arizona. 



quartz are present in about equal proportions, but their total 

 amount does not equal that of the plagioclase. Brown biotite 

 is present in somewhat less quantity than the hornblende. 

 Titanite and magnetite occur as accessories. Occasionally the 

 quartz is poikilitic in the orthoclase. The feldspathic and fer- 

 romagnesian minerals are present in about equal proportions. 

 If it were not for the preponderance of the plagioclase the 

 rock might be classed as a quartz-monzonite or grano-diorite. 

 It is probably best classed as a quartz-diorite with a monzo- 

 nitic aspect. The microscope reveals no cataclastic structure 

 nor other evidence of dynamic action, and the minerals are 

 fresh and unaltered. 



The pegmatites are pink in color and usually very coarse in 

 texture. They are composed chiefly of quartz and pink ortho- 

 clase. Some of the dikes carry large crystals of silvery- white 

 mica. No other minerals were noted. The dikes usually 

 exhibit the typical comb-structure inward from the walls and 

 the graphic arrangement of the quartz and feldspar. Along 

 the walls of some of the dikes the texture becomes aplitic. 



Origin. — There is no clear evidence in the Shinumo area by 

 which the original character of the schists of the Yishnu 

 series can be determined. There is no evidence of a banding 

 that can be clearly referred to original sedimentary bedding, 

 nor is there evidence of any original clastic texture. The 

 mineralogical composition, however, suggests a sedimentary 

 origin for the quartz schists of the mica and hornblende type : 

 either might have resulted from the regional metamorphism of 

 an arkose sandstone or shale. According to the lesser or 

 greater abundance of iron in the original sediments the rock 

 wouid assume the mica or hornblende type of quartz schist. 

 Certainly the great preponderance of quartz as a constituent 

 of these rocks would seem to weigh against an igneous origin 

 in the balance of probability. The causes operating to pro- 

 duce the schists in their present aspect are conceived to be 

 the processes connected with regional metamorphism, namely, 

 subsidence and deep burial, subsequent folding and mashing, 

 and a slow recrystallization in process of time. 



The original character of the hornblende schist described as 

 occurring in a narrow outcrop between the schists is not clear. 

 The fact that the rock consists of little else than hornblende 

 suggests that it originally constituted an igneous rock of a 

 basic type. The fact that the microscope discloses a schistose 

 structure is evidence that the rock is at least earlier in age 

 than the period of regional metamorphism that was responsible 

 for the present structural and mineralogical character of the 

 enclosing schists. 



The origin of the quartz-diorite is reasonably clear. Since 



