MOOK, STUDY OF THE MORRISON FORMATION 67' 



qiiartzite, 50 to 100 feet thick, usually in a single bed. Above this is a 

 blue limestone, which contains shells of Limnea, Valvata and Cypris. 

 In some cases this limestone is succeeded by more sandstones, and in 

 other cases these sandstones are absent. The upper part of the formation 

 consists of gray, drab, pink and purple clays and marls, through which 

 run thin intermittent beds of drab limestone. 



The lower part of this formation may correspond to the La Plata sand- 

 stone rather than to the McElmo or Morrison. It is quite probable, 

 however, that most of the formation is equivalent to the latter. 



This locality is about midway between the eastern front of the Kocky 

 Mountains and the areas west of the mountains, where the Morrison has 

 a great thickness. The 23resence at this point of Morrison beds of medium 

 thickness indicates the probable former extension of the deposits across 

 the country now occupied principally by the crystallines of the Eocky 

 Mountains. 



Peale (1877, 2) describes "Jurassic shales" in San Miguel and Dolores 

 canyons and in the Uncompahgre Valley. The creeks tributary to the 

 Gunnison cut through Dakota and soft "Jurassic" shales into the under- 

 lying red sandstones. The San Miguel cuts through "Jurassic shales." 

 The following section on a creek tributary to it is given 1:)y Peale : 



1). Upper Dakota sandstone. 



2). Lower Jurassic shales. 



3). Jurassic variegated beds. 



4). Massive red sandstone, light colored. 



At one point the "Jurassic shales" rest on the gneiss, according to 

 Peale. He discusses the "Jurassic shales" as follows : "Immediately 

 above the red beds is a group of shales and marls, with thin bands of 

 limestone near the base. These beds are variegated in color, and corre- 

 spond, lithologically and stratigraphically with the beds that, in eastern 

 and central Colorado, I referred to the Jura. . . . They appear to 

 correspond closely with the beds measured in the section on the Gunnison 

 in 1874." 



The Gunnison formation of Eldridge was divided by Cross in tbe 

 Telluride folio into two formations, the La Plata formation corresponding 

 to the lower part of the Gunnison formation of the Crested Butte section, 

 and the McElmo formation corresponding to the upper part of the orig- 

 inal Gunnison. The McElmo corresponds much more closely with the 

 eastern Morrison than the La Plata, but in some localities it is difficult 

 to separate the La Plata from the McElmo, and it is possible that some 

 portions of the La Plata are represented in the eastern Morrison. 



