3(>8 Schu chert — Cambrian of the Grand Canyon of Arizona . 



Tapeats sandstone Walcott 4 has gotten Obolus chuaren- 

 sis and the trilobites Alohistocare althea, Dolichometo- 

 pns productus and D. tontoensis. The two last named 

 trilobites are typical Middle Cambrian forms of the Cor- 

 dilleran province, and as guide fossils have far more 

 value than any of the brachiopods. 



Upper Cambrian Series. — The top member of the 

 Tonto series is Noble's Muav limestone (fig. 4). To the 

 west in the Shinumo area the Upper Cambrian series is 

 more calcareous than in that of the Bright Angel and 

 therefore it can there be properly referred to as a lime- 

 stone formation. 5 About El Tovar, however, the series 

 is best referred to as the Muav formation. In the Her- 

 mit camp area the thickness is, according to Noble, 425 

 feet, though in his Geologic History of the Bright Angel 

 Quadrangle, 1914, it is given as 380 feet. The thinning, 

 Doctor Noble writes me, is not due to erosion, but to 

 actual shrinkage in the thickness of the beds as they are 

 traced from west to east. Throughout the Grand Canyon 

 region occur many isolated hollows eroded into the Muav, 

 in which are preserved fresh-water sandy limestones up 

 to 100 feet in thickness, bearing locally fish remains of 

 Upper Devonian age. Above the Hermit trail in Cope 

 Butte there is such a hollow, 80 feet deep, inset into the 

 Upper Cambrian formation. The Muav makes steep 

 slopes in the lower half and cliffs above, due to the pro- 

 tecting hard limestones of the Redwall, which are of early 

 Mississippian age (fig. 4). 



In general the Muav formation about El Tovar may be 

 defined as a buff colored series of decidedly variable cal- 

 careous strata, or as a series of impure micaceous (mus- 

 covite) shaly and sandy limestones (70%), calcareous 

 shales (18%), and calcareous sandstones (12%). It is 

 only in the uppermost 50 feet that the strata may be 

 regarded as more or less pure limestones. Where the 

 calcareous materials become more dominant the beds are 

 completely riddled with vertical and anastomosing worm 

 burrows. These are usually filled with a very fine sand 

 and it is this feature that gives so much of the Muav its 

 mottled appearance and that led Gilbert many years ago 

 to call the formation in the western part of the Grand 

 Canyon the ' ' Mottled limestone. ' ' Throughout the mid- 



* Walcott, Smithson. Misc. Coll., vol. 64, pp. 369-374, 184, 1916. 

 6 L. F. Noble, U. S. Geol. Survey, Bull. 549, p. 64, 1914. 



