The Shinumo Area. 385 



it is a coarse-granular, igneous rock of plutonic aspect occur- 

 ring over a large area, clearly cutting the schists, and showing 

 no textural modifications at the contact, it is concluded that 

 it represents a deep-seated igneous invasion of considerable 

 size, of the type known as a " batholith." Because of the 

 unaltered character of the rock and the absence of any gneis- 

 soid or cataclastic structure, it is argued that the batholithic 

 invasion took place at a time later than the period of regional 

 metamorphism that produced the recrystallization and schistos- 

 ity of the enclosing schists. The question may arise as to 

 whether the invasion of the batholith was not of itself a par- 

 tially operative cause in producing this recrystallization and 

 schistosity. The held evidence is against such a conclusion : 

 there is no gradation in the schist away from the contact, 

 either in texture or in mineral composition, nor are the planes 

 of schistosity parallel to the contact. 



The older pegmatite dikes are folded intimately with the 

 schists. Their injection may have either preceded or accom- 

 panied the regional metamorphism. 



The younger pegmatite dikes cut both the schists and the 

 quartz-diorite. Where they cut the schists they break cleanly 

 across the schistosity. The injection of these dikes is the latest 

 visible event in the igneous activity of Archean time within 

 the area. 



Age and correlation. — The rocks of the Vishnu series are 

 rocks which, in the light of present knowledge, can only be 

 conceived to have acquired their present character at great 

 depths beneath the earth's surface in what is technically known 

 as the " zone of flowage." It is therefore evident that the 

 unconformity which separates them from the overlying Algon- 

 kian sediments of the Grand Canyon series represents a vast 

 amount of erosion and the consequent lapse of an enormous 

 interval of time, — an interval vastly greater even in events 

 than that represented by the profound unconformity which 

 separates the succeeding Grand Canyon series from the over- 

 lying Paleozoic. The Vishnu schists are therefore assigned to 

 the Archean in the usage of the United States Geological 

 Survey ; but whether as a group or a complex is as yet unde- 

 cided. It seems likely, as stated by Ransom e (a, p. 21), that 

 they are to be correlated with the Pinal Schist of the Globe 

 and Bisbee regions, and " present somewhat different aspects 

 of the fundamental crystalline complex of Arizona." 



Up to the present time no detailed study of these rocks has 

 been made in the Grand Canyon region, and their internal 

 structural relations are unknown. It is not unlikely when 

 such a study is made of their exposures in the Kaibab division 

 of the canyon, in the Shivwits division to the west, and along 



Am. Jour. Sci.— Fourth Series, Vol. XXIX, No. 173.— May, 1910. 

 26 



