RESUME OF LITEEATUEE. 123 



The Ouray limestone, is made up of massive beds of limestone, separated by thin 

 intercalations of marl or shale. Fossils are to be found in most sections, and show 

 the geologic age to be upper Devonian. Only the Devonian fauna was known at 

 the time this report was written, but at certain localities, at least, the top strata of 

 the Ouray limestone contain fossils of a distinctly Mississippian aspect. The same 

 Devonian fauna occurs in the Leadville limestone at Glenwood Springs, in the vicinity 

 of Salida, and in the Ci'ested Butte quadrangle, while a similar Mississippian fauna 

 is found in the upper portion of the Leadville at Salida, in the Crested Butte quad- 

 rangle, and at Aspen. It can be regarded as established, therefore, that the Ouray 

 limestone is the equivalent of the Leadville limestone of the Elk Mountains. 



The Carboniferous of Cross and Spencer consists of the Hermosa and Rico 

 formations, both of Upper Carboniferous age, for the Mississippian fauna of the 

 Ouray limestone was not known at the time of their writing, and the Mississippian 

 epoch accordingly was supposed to be unrepresented. On the evidence of the 

 invertebrate faunas the Hermosa formation is referred to the Pennsylvanian period, 

 and the Rico formation to the Permo-Carboniferous. 



"Occurring between the Devonian limestone and the typical Red Beds, and 

 sharply defined from each, there is a heterogeneous series of rocks which is gen- 

 erally distributed in the San Juan region, where it reaches a maximum thickness of 

 about 2,000 feet * * * Lithologically the Hermosa is composed of limestones, 

 shale, and sandstone, but all of these strata are more or less calcareous throughout. 

 The limestones are of a blue-gra}' color, rather dense in texture, and usually very 

 fossiliferous. They are frequently more or less bituminous, sometimes so much so 

 as to aii'ord a distinct odor of petroleum when struck with a liammer.' The shales 

 vary from black bituminous clay shales, rather fissile, to sandy shales and sand- 

 stones of an olive-green color. The sandstones are also of a greenish color * * * 

 This formation has a considerable distribution in the western part of the San Juan 

 region, but throughout the area of its occurrence it is not in general divisible, since 

 individual beds and groups of strata change greatl_y in chai-acter from place to place, 

 so that horizons can not be definitely recognized in localities separated from one 

 another by more than short distances." 



At Rico, still more than in the Animas section, are the limestones persistently 

 segregated in the middle portion of the formation. It is therefore possible to 

 divide the Hermosa at Rico into three approximatelj^ equal parts. The Hermosa 

 formation corresponds in a general manner with the Middle Carboniferous mapped 

 by the Hayden survey. 



Between the Hermosa formation and the Ouray limestone in the Needle Moun- 

 tains quadrangle occurs a bed of red shaly material whose character and occurrence 

 are peculiar. It was not given particular description in the report by Cross and 

 Spencer, but it is now proposed to discriminate it as the Molas formation. The 

 following account of this horizon is abstracted from a description written hj Mr. 

 Cross: 



