The Shinumo Area. 381 



General Geology. 

 Archean — Vishnu Schist. 



Name. — The name " Vishnu Terrane " has been given by 

 "Walcott (Walcott, c) to the fundamental crystalline com- 

 plex of the Grand Canyon region that underlies the unaltered 

 sedimentary rocks of Algonkian age, and is separated from 

 them, as well as from the overlying Cambrian, by a profound 

 unconformity. The type locality is situated on the Colorado 

 River thirty miles east of the Shinumo area at the base of one 

 of the great buttes called " Vishnu's Temple," from which he 

 derives the name. 



Distribution in the Kaibab division. — The rocks of the 

 Vishnu series are exposed continuously for a distance of over 

 forty miles within the Kaibab division of the Grand Canyon in 

 the bed of the Colorado River and in the walls of its gorge 

 which is cut beneath the Tonto platform. The Shinumo area 

 is at the western end of this long exposure. The eastern limit 

 of the exposure is determined by the appearance of great 

 thicknesses of overlying strata of the Grand Canyon series 

 dipping northeastward into the bed of the river, in the classic 

 locality described by Walcott. The western limit of the 

 exposure is likewise defined by a structural cause : three miles 

 west of the Shinumo area a southward bend of the river 

 causes it to flow parallel to the gentle southwestward dip of 

 the Paleozoic rock system of the canyon wall, carrying the bed 

 of the stream up out of the schists and into the basal Tonto 

 sandstone of the Cambrian. 



Occurrence and distribution in the Shinumo area. — The 

 Vishnu schists are exposed for a distance of three miles in 

 the gorge of the Muav-Flint Creek Canyon. They are exposed 

 in the bed of the Colorado river throughout its course across 

 the Shinumo area. Along the south bank of the river for a 

 distance of four miles, and along the north bank for a distance 

 of seven miles, in the eastern half of the area, sediments of 

 the Grand Canyon series intervene between the Vishnu schists 

 and the Tonto sandstone of the basal Cambrian. Elsewhere 

 the schists are exposed without break in the walls of the river 

 gorge beneath the Tonto sandstone. 



Iso detailed study of these rocks was made in the Shinumo 

 area beyond the general location and determination of the vari- 

 ous types represented, since the limited extent of the expo- 

 sures precludes the possibility of unraveling any general 

 Archean structure from the study of this area alone. They 

 are here a metamorphic complex of quartz, mica, and horn- 

 blende schists, invaded by a batholithic mass of quartz-diorite, 

 and injected by veins of pegmatite and aplite. 



