W1LUAMS.] HAGUE, COMSTOCK. 223 



The boundary line between the Silurian and Devonian is said to be 

 arbitrarily drawn, as the passage from the lower to the upper limestone 

 is gradual, « with poorly defined lithological distinctions, and without, 

 as yet, any paleoutological evidences " for making sharp distinctions. 1 

 But below the Lone Mountain limestone (Silurian) is a plane of uncon- 

 formity. 



The Nevada limestone, although so thick (6,000 feet), offers no litho- 

 logic or paleontologic evidence by which to divide it sharply into sub- 

 divisions. The fauna is rich and often well preserved, and contains 

 species of the Upper Helderberg, Hamilton, and Chemung formations 

 of New York. While there is recognized a lower and upper fauna, 

 many of the species show a remarkable range, and some of them " have 

 reversed their relative positions in the group as they have been known 

 heretofore. Among the Brachiopods Orthis tulliensis, of the Tully 

 limestone of New York State, is found at the summit of the Devonian 

 limestone, and Orthis impressa, a Chemung species of New York, at the 

 base, associated with eastern Upper Helderberg limestone species." 2 



The White Pine shale, in the White Pine district, carries a fauna 

 which combines species ranging from Middle Devonian to Lower Car- 

 boniferous in the east. The Devonian fauna described contains 102 

 genera and 225 species, and 94 genera and 79 species of these are iden- 

 tified as common to Nevada and New York. Two species described 

 from the Mackenzie River Basin were identified among the Eureka 

 Devonian fossils. The Carboniferous age of the Diamond Peak Quartz- 

 ite is determined by the occurrence of a Carboniferous Productus in an 

 intercalated limestone stratum 500 feet from its base. The lower lime- 

 stone contains evidence of proximity of land in the presence of frag- 

 ments of plants and pulmoniferous mollusks, but the fossils throughout 

 the carboniferous deposits of Nevada are of marine species, and no 

 beds of coal occur in them. The whole series of formations of the 

 upper Paleozoic presents strong contrast to anything seen in the east- 

 ern part of the continent, and the stratigraphy as well as the paleon- 

 tology furnishes striking example of the unreasonableness of attempts 

 to unify the geologic classifications of the world. 



Mr. T. B. Comstock 3 in 18S3 reported on the rocks of San Juan 

 County, Colorado. 



The Devonian rocks of this region are " exposed near the summit of 

 the divide between Bear Creek and Cascade Creek and along a line 

 running parallelwise with the Animas Canon, forming the cliffs along 

 the side of Lime Creek." The outcrop occurs again at Silverton and 

 near the head of Cunningham Gulch. Although the Devonian is not 

 sharply distinguished from the rocks below, the fossils in the upper 

 part of the limestone point definitely to a Devonian horizon. 



'Abstract of report, etc., p. 265. 

 * Walcott: Paleontology of the Eureka District, p 4. 



■ Comstock, T. B. : Notes on the Geology and Mineralogy of San Juan County, Colorado, Trans. 

 Amer. Inst. Min. Eng., vol. 17, 1883, pp. 165-191. 



