Gilbert] FLAGSTAFF TO THE GRAND CANYON. 473 



The Aubrey limestone withstands erosion so much better than the 

 Shinarump (Permian) shales and sands above it, that its surface has 



been denuded over a vast area and constitutes the floor of the country. 

 Each great otogenic block stands as a plateau, each fault is marked 

 by a cliff, each rock flexure is revealed in a topographic profile. Throngh 

 this grand tectonic model runs the river's trench, revealing its anatomy 

 in either wall. 



The brink of the chasm is reached at a point nearly opposite Point 

 Sublime, and the view does not differ in character from that sketched by 

 Holmes.' 91 '- 94 The canyon is here broad and its walls are elaborately 

 sculptured in sinuate terraces and cliffs, with buttresses, alcoves, pyra- 

 mids, and spires innumerable. The Aubrey limestone and a linn sand- 

 stone beneath it, both pale in tint, constitute the first cliff, and a broad 

 sloping terrace below it reveals a series of bright red shales and sand- 

 stones likewise of the Aubrey group. The foundation of this ter- 

 race and the material of the next cliff is a massive gray limestone, 

 named the Red Wall because generally stained by pigment washed 

 from above. The cliff is 1,000 feet (300 m.) high, and can be scaled only 

 here and there in a deep recess. The next terrace is due to sandy shales 

 of dingy hues, green, gray, and brown— the Touto shales; and the Tonto 

 sandstone forms a chocolate-colored cliff beneath. 



The Aubrey beds and the Red Wall limestone carry Carboniferous 

 fossils; there are Cambrian fossils in the Tonto shales and Tonto sand 

 stones. Devonian and Silurian claim a narrow zone at the base of 

 the Red Wall. Beneath the Tonto sandstone is a profound unconfor- 

 mity; it rests partly on Archean schists and granite, partly on basset 

 edges of two great systems of Algonkian strata, comprising all ordinary 

 types of clastic rocks, but yielding only tantalizing traces of contem- 

 porary organisms. These systems are themselves separated by an 

 unconformity, and a still greater break divides the lower from the 

 Archean. 



