382 JVoble — Geology of the Grand Canyon, Arizona. 



Five main types of rock are found in this complex within 

 the area : 



The first type is a quartz schist, with graduations into mica 

 schist. It comprises the greater part of the Yishnu schists 

 that are exposed iu the gorge of the Colorado River west of 

 the cable crossing at Bass Ferry. It is also exposed in the 

 bed of Muav Canyon, and in the bed of Flint Creek above the 

 junction with the Shinumo. 



The second type is a quartz schist with gradations into 

 quartz-hornblende schist. It is exposed in that part of the 

 Muav-Flint Creek Canyon that is occupied by the living stream 

 of the Shinumo, grading both eastward and westward into the 

 quartz-mica schist. 



The third type is a hornblende schist. It occurs in one 

 small outcrop about 200 feet wide, on the east side of " Fault 

 wash," a dry wash which joins the Shinumo just below the 

 mouth of Muav Canyon, and is sharply bounded on both sides 

 by the quartz-mica schist. 



The fourth type of rock is a quartz-diorite. So far as 

 observed, it constitutes all of the Yishnu series of the Shinumo 

 area that is exposed in the " granite gorge " of the Colorado 

 River east of the cable crossing. Its western contact is well 

 defined. The eastern limit of the exposure was not located. 



The fifth type of rock is a granitic pegmatite which occurs 

 in dikes that cut all the rock types of the Yishnu. These 

 dikes occur in two generations. The older generation is folded 

 with the schists. The younger generation occurs in a great 

 network or mesh of dikes cutting both the quartz-diorite and 

 the schists. 



The Yishnu schists have a typical schistose structure. The 

 planes of schistosity have a dip that seldom departs greatly 

 from the vertical. The dominant trend of the schistosity is 

 northeasterly, but varies, from place to place. Locally, the 

 schists are much twisted and contorted. 



Lithology. — The less micaceous phase of the quartz-mica 

 schist has a dark grey color with a greenish tinge. The cleav- 

 age is imperfect and the surfaces have a satin-like luster. 

 The texture is fine-grained and the unaided eye can distinguish 

 no mineral constituents except quartz/ The microscope shows 

 the rock to be composed almost entirely of fine interlocking 

 grains of quartz, with occasional small flakes of white mica, 

 arranged in parallel lines. In the extreme quartzose phase 

 the amount of mica is almost negligible, just enough being 

 present to impart a satin-like luster to the rock. 



The micaceous phase of the schist has a slight grey color 

 with either a pinkish or greenish tinge on the fresh fracture. 

 Where the rock is weathered, the color becomes red. The 



