The Shinumo Area. 517 



proportion of sand in that division of Walcott's section which 

 corresponds to the third member on the Shinumo. The succeed- 

 ing members correspond closely in character and thickness 

 even to the minor divisions ; an example of this is the 

 "gnarled and twisted layers" previously cited. 



The writer had the privilege of examining Mr. Walcott's 

 field specimens in the National Museum in Washington and 

 was particularly impressed by their absolute lithological iden- 

 tity with the series collected by himself from corresponding 

 horizons on the Shinumo. The two series of specimens, with 

 the exception of those rocks altered by local metamorphic phe- 

 nomena, might have come from the same locality. 



Diabase Intkusive in the Unkar. 



Occurrence. — The diabase occurs in the form of an intrusive 

 sheet or sill which occupies three separate stratigraphic hori- 

 zons in the Unkar sediments in different parts of the Shinumo 

 area. On the west side of the Asbestos Canyon three miles 

 west of the Shinumo it is intrusive in the limestones of division 

 c of the second member of the Unkar at a horizon 15 feet above 

 their base. On the east side of the Asbestos Canyon it ascends 

 out of this horizon and breaks across the overlying limestones 

 and shales in an eruptive contact. Most of the lower, and part 

 of the upper, sections of the eruptive contact are beautifully 

 displayed in the walls of this canyon, but the complete expo- 

 sure is obliterated above by the truncating unconformity 

 at the base of the Tonto sandstone. Between the exposures in 

 the Asbestos Canyon and the main areal exposures about the 

 mouth of the Shinumo, the pre-Cambrian structure is hidden 

 beneath the sandstone of the Tonto platform. Where the dia- 

 base reappears in the exposures about the Shinumo it is found 

 to lie intruded midway within the shales of the third mem- 

 ber of the Unkar at a horizon 400 feet above that which 

 it occupies in the Asbestos Canyon. This is the stratigraphic 

 position which it holds in all the exposures of the central part 

 of the Shinumo area. Eastward up the river from the East 

 Wash the diabase again disappears beneath the pre-Tonto un- 

 conformity. But directly under Havasupai Point, about three 

 miles farther east, limited outcrops of the basal portion are 

 exposed in small inter-canyon valleys on both sides of the river. 

 Here the lower contact of the diabase lies just at the top of 

 the " white limestones" of division c of the second member 

 of the Unkar. 



The thickness of the sill is approximately 650 feet on the Shi- 

 numo and 950 feet on the west side of the Asbestos Canyon. 

 Eastward from the Shinumo the outcrop is considerably 



