the Muddy Mountains, Nevada, etc, 51 



well rounded. Beds are local and lenticular^ and the en- 

 tire formation is characterized by extreme cros shedding. 

 Fragments of silicified wood are abundant, and large logs 

 are common, especially near the top. Descriptions of the 

 conglomerate in the Navajo country, 300 miles to the 

 east, are equally accurate for the formation in the Muddy 

 Mountains. With the exception of the petrified wood no 

 fossils were found, and the conglomerate is called Upper 

 Triassic merely to agree with its present classification in 

 the Colorado Plateaus. 



The Shinarump conglomerate is overlain, without ap- 

 parent unconformity, by the Chinle formation, a thick 

 series of sandstones and gypsiferous shales. Near Colo- 

 rado River this formation is 800 feet thick, and at least a 

 third of the thickness is made up of coarse sandstone lay- 

 ers, many of them conglomeratic. Northward the thick- 

 ness increases to 3,000 feet within 20 miles, and the coarse 

 materials almost disappear. Near Muddy Creek the 

 greater portion of the formation consists of red gritty 

 shale and fine-grained sandstone, with an abundance of 

 gypsum, both primary and secondary. Eipple-marks are 

 common, and silicified wood occurs at some horizons. 

 Evidently the deposits are continental, and the source 

 was in highlands which lay to the south, probably in cen- 

 tral or southern Arizona. The formation corresponds 

 closely in lithologic character and in stratigraphic rela- 

 tions to the Chinle formation of the Navajo country and 

 to the Dolores of southwestern Colorado; and in this 

 report the name Chinle is extended to the horizon in the 

 Muddy Mountains, although no faunal evidence of its age 

 was found in that region. 



Jurassic System. — No unconformity was found between 

 the Chinle formation and the overlying Jurassic sand- 

 stone, which is heavy bedded, cross-bedded on a large 

 scale, and typically bright red, although gray is the pre- 

 dominant color locally. As a rule the false beds are 

 straight and meet the true bedding planes at high angles ; 

 but tangential cross-bedding also occurs, and in some sec- 

 tions it is very conspicuous. The sand grains are fine to 

 medium in size and show indications of wear by the wind. 

 Probably both wind and water had a part in the final 

 deposition of the sand. Without doubt the formation 

 was laid down in an arid climate, and its great thickness 

 — ranging from 800 to at least 2,000 feet — required neigh- 



