The Shinumo Area. 499 



structurally a unit with the exposures of the Shinumo area. 

 It is unmentioned in geological literature. 



In the western end of the Kanab division in a section across 

 the Grand Canyon at the foot of Toroweap valley, 50 miles 

 west of the Shinumo area, Dutton figures " rocks of Silurian 

 and Archean unconformable" in the bed of the river beneath 

 the basal .Tonto sandstone of the Cambrian (Dutton, a, p. 88). 

 It is probable that these " Silurian" rocks there represent the 

 Grand Canyon series. 



Whether these rocks appear at places in the Shivwits 

 division between the Vishnu schist and the Tonto sandstone 

 is not at present known. 



Stratigraphic position, structure, and distribution in the 

 Shinumo Area. — Two unconformities determine the strati- 

 graphic position of the sediments of the Grand Canyon series 

 in the Shinumo Area : they are separated from the underlying 

 Vishnu schists of the Archean by a profound unconformity 

 which represents a base-leveled surface of erosion, and from 

 the overlying Tonto sandstone of the Cambrian by an uncon- 

 formity which represents a similar base-leveled surface above. 



The strata of the Grand Canyon series here constitute a 

 wedge-shaped mass whose apex lies along the south side of 

 the Colorado river parallel to its northward course in this 

 part of the area. The mass as a whole constitutes one great 

 tilted block, which in turn consists of a great number of minor 

 tilted and rotated blocks pitching at successively greater angles 

 northeast away from the apex of the wedge, until at a distance 

 of three miles from the Colorado river the whole mass is 

 dropped by a profound fault which, brings up the under- 

 lying Vishnu schist from a great depth, — a structure which 

 strikingly resembles that of the area of similarly faulted 

 Triassic blocks of the Connecticut valley. The strike of the 

 strata is !N". 40° W. The dips are variable : in general the 

 strata of the fault blocks near the apex of the wedge dip 

 10°-15° N.E. ; near the center of the wedge the dips average 

 25° N.E. ; while in proximity to the line of the great limiting 

 fault on the northeast they are completely reversed by the 

 "drag" along the fault plane. The truncation of this pre- 

 Cambrian structure by the unconformity at the base of the 

 Tonto sandstone is absolute. 



The great pre-Cambrian fault that limits the wedge upon 

 the northeast represents the exposure in the basement rocks of 

 the line of displacement of the West Kaibab fault and displays 

 in the most spectacular manner a phenomenon analogous to that 

 described by Walcott upon the line of the East Kaibab mono- 

 cline (Walcott, a). Upon the line of the ancient fault in the 

 Shinumo area two later displacements have taken place after 



