NORTH AMERICAN GEOLOGY. 219 



Comstock describes the geology and vein structure of southwestern 

 Colorado, and discusses at some length the vein stones and ore deposits. 

 The following is an abstract of the statements in regard to the surface 

 ■ geology : The crystalline rocks consist of a quartzite group, and an 

 overlying granite, which are considered post-Archean, and possibly 

 Lower Paleozoic. The Silurian and Devonian are represented at some 

 points by fossiliferous rocks, and may occur over a wide area, of which 

 the age is now in doubt. The Carboniferous is divided into two series, 

 the earlier represented by 1,200 feet or more of argillaceous, arenaceous, 

 and calcareous beds, the latter by over 1,200 feet of red sandstone, 

 with occasional local trachytic inclusions. The Triassic and Jurassic 

 do not cover any considerable area, and may be absent. The Cretace- 

 ous appears in force, and the series 1-5 of the Hayden survey are rec- 

 ognized. Coal-beds are found in the upper part of the upper Dakota. 

 The Tertiary is represented by Eocene lake beds, and great lava flows, 

 which appear to be chiefly middle Tertiary. Five series of volcanic 

 eruptioDs are recognized in the following order: Propylite, andesite, 

 trachyte, crater flows of rhyolite, and finally a few patches of basalt, 

 probably the remnants of Pliocene flows.* 



87. In the Red Mountain districts great geyser deposits are found, 

 and it is supposed that at one time this area experienced geyser action 

 similar to that now active in the Yellowstone Parkt 



88. Stephens gives a general account of observations made on a trip 

 through the San Juan Mountains. The underlying rocks of the district 

 are a crystalline series, generally overlain by Silurian sandstones, 

 quartzites, etc., and these in turn are often overlain by the Carbonifer- 

 ous. Structural features and outcrops are generally obscured by great 

 masses of volcanic rocks. North of Ouray the country is traversed by 

 numerous faults, often of great magnitude. Two periods of volcanism 

 are recognized, both subsequent to the last uplift. The first is termed 

 the porphyry period, and its viscid lavas are found overflowing the 

 whole district, and intruded between many of the beds, causing great 

 alteration. The second is termed the trachytic period, in which an ex- 

 ceedingly viscid lava appears to have been outpoured upon the moun- 

 tain tops, but was not sufflciently liquid to flow into the valleys. | 



89. In his report as geologist of Wyoming for 1885, Aughey discusses 

 the geologic position of the hematite deposits of Seminole Mountain. He 

 considers the ore-bearing series to be of approximately Huronian age. 

 It consists of quartzites, and gneissic, hornblendic, and chloritic rocks, 

 with limestones, jasper, and epidotic slates, which form the core of the 

 mountain, and are overlain by Potsdam, Carboniferous and Cretaceous 

 in succession. § 



* Trans., 1886. 



t American Naturalist, vol. 20, pp. 963-965. 



tNew York Acad. Sci., Trans., vol. 5, pp. 121-130. 



^Wyom. Geolog. Rep., pp. 120. 8vo. Laramie, 1886. 



