364 INDEX TO THE STKATIGEAPHY OF NORTH AMEEICA. 



The Robinson formation, comprising sediments and trachytic tuffs. This contains fossils 

 showing the age to be upper Carboniferous. The formation is known on the gold belt series 

 of maps only in the Downieville quadrangle, a short distance south of the fortieth parallel. 



The Calaveras formation, comprising by far the largest portion of the Paleozoic sediments 

 of the gold belt. Rounded crinoid stems, corals (Lithostrotion and Qisiophyllum), Forami- 

 nifera (Fusulina), and bivalves have been found in the limestone lenses and indicate that a con- 

 siderable .portion at least of this formation belongs to the middle or lower Carboniferous. In 

 extensive areas of the Calaveras formation no fossils have, however, been found, and older 

 rocks may be present in these. It is not likely that post-Carboniferous rocks are present in 

 these nonfossiliferous areas. 



In the Colfax quadrangle, which lies in the northern-central part of the Sierra 

 Nevada (latitude 39°-39° 30', longitude 120° 30'-121°), the sedimentary rocks of the 

 "Bedrock series" are divided by Lindgren^^ into three "groups," of which the oldest 

 is "the Carboniferous group equivalent to the Calaveras formation of other fohos. 

 * * * This group can be subdivided into five formations, hthologically very 

 distinct, though the fossils do not afford data for paleontologic discrimination. 

 These are enumerated from east to west as follows : Blue Canyon formation, Relief 

 quartzite, Cape Horn slates, Delhi formation, and Clipper Gap formation." 



According to Lindgren the principal Carboniferous area in the quadrangle is 

 that of his Blue Canyon formation, occupying the southeast corner, adjacent to 

 "Jura-Trias" (Sailor Canyon) rocks, and intruded by granite. The rocks are black 

 fissile clay slates and fine-grained quartzitic sandstones, with one occurrence of fine 

 conglomerate and a few limestone lenses. At two localities these limestones con- 

 tain fossils (Lithostrotion, Aviculipecten, a lamellibranch, and Miirchisonia at one 

 place; SjTingopora, Diphyphyllum or Lithostrotion, and crinoid stems at the other). 

 These suffice only to indicate a Paleozoic age. 



Lindgren described his Relief formation as a very hard, fine-grained quartzite, 

 alternating with streaks of siliceous clay slates, plainly exhibiting the stratification 

 but unfossihferous; his Cape Horn formation as consisting of typical clay slates, 

 fissile, almost black, weathering to gray, and containing small limestone lenses which 

 have yielded round crinoid stems of Paleozoic age; and his Delhi formation as com- 

 posed of a peculiar dark-brown or black hard rock, resembling hornfels (a character 

 probably due to metamorphism of siliceous sediments of a certain kind), with a few 

 lenticular masses of limestone and round crinoid stems. 



Lindgren's Clipper Gap formation consists of black clay slates and argillaceous 

 sandstones. Lenticular bodies of limestone are abundant and chert is common. 

 Obsciire fossUs (PhiUipsastrea and Pleurotomaria) were obtained at one locality, and 

 others (Clisiophyllum gdbbi Meek, Lithostrotion whitneyi, and brachiopod frag- 

 ments) of lower Carboniferous age were found in a disconnected mass that is 

 probably a fragment of the formation torn loose by a diabase eruption. 



J, 11. EASTERN NEVADA. 



From Hague's very full account '''^'' of the Carboniferous section at Eureka, Nev., 

 the following extracts are taken: 



At Eureka the Carboniferous rocks have been estimated to measure 9,300 feet in thickness, 

 which, however, does not represent the full development of the Carboniferous period, the upper 

 Coal Measures, the top of the Paleozoic system, having suffered a very considerable amount 

 of erosion. This upper limestone is by no means so thick as that found elsewhere. 



