102 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



The oldest Cambrian fauna now known, with Nevadia weeksi and Holmia rowei, is limited 

 to a few forms, but with a careful examination of the region where it occurs in southwestern 

 Nevada it is highly probable that a considerable fauna will be found. The strata in which 

 it occurs were deposited in a depression opening out toward the Pacific Ocean, where south- 

 western California is now located; this depression soon extended northward and presumably 

 connected through to British Columbia and Alberta, as the same species of OleneUus occur 

 in the central and upper portions of the Lower Cambrian both in Nevada and Alberta. 



I do not know of a Cambrian fauna as old as that of Nevadia weeksi on the eastern side of 

 the continent, or on the European continent. It. appears to be a portion of the older fauna 

 that is missing everywhere except in southwestern Nevada. I think it was brought in by 

 the advancing Lower Cambrian sea from a sea to the west, the sediments of which are buried 

 beneatla later strata or are off the present shore line of the continent beneath the sea. 



The theory that life originated and developed in fresh-water ponds and lakes does not 

 appeal to me. More uniform conditions of temperature and environment would be present 

 in the ocean and the sediments of the fresh-water deposits of pre-Cambrian and Cambrian 

 time, if such exist, do not show sufficient evidence of life having existed at the time of their 

 deposition. The Algonkian fossils of the Belt and Grand Canyon series (Walcott, 1899, pp. 

 227-239) probably came from the marine fauna when a temporary coimection existed between 

 the interior fresh or brackish water lakes and the ocean. 



Spurr"^ gathered in one volume the observations made by himself and others 

 on the occurrences of Cambrian strata and faunas in 25 widely distributed ranges 

 of the Great Basin. Ball ^* supplemented Spurr's descriptions for southwestern 

 Nevada and eastern California and gave a columnar section showing the relations 

 of the Paleozoic strata among themselves and to the post-Carboniferous and 

 Tertiary igneous rocks. 



Darton ^^^ has noted the occurrence of Cambrian strata resting on older granite 

 at Iron Mountain, in eastern San Bernardino County, Cal. He observed 1,200 

 feet of shale and limestone with quartzite at the base and collected "fragments 

 of trilobites and moUusks which proved not to be specifically determinable," but 

 which according to Walcott are "undoubtedly Cambrian, probably Middle 

 Cambrian." 



J 13. COLORADO. 



The Upper Cambrian is represented in southwestern Colorado in Animas 

 Valley by the Ignacio quartzite, a basal deposit, unconformable on the Algonkian. 

 Cross ^** describes the Ignacio as follows : 



The lowest lithologic division of the Paleozoic section in the Animas Valley is made up 

 of quartzites and varies in thickness, in the area thus far examined, from a few feet to 200 

 feet.. In layers near the middle of ^lese quartzites a single genericaUy determinable shell 

 has been found. From the stratigraphic relations and the evidence of this fossil it is assumed 

 that in this region the Cambrian system is represented only by a thin series of quartzites belong- 

 ing probably to its upper division, and for those the name Ignacio formation or quartzite is 

 proposed, from the lakes in the Animas Valley about 18 miles west of south from Silverton, 

 near which the formation is well exposed. It is at present believed that alL of the Paleozoic 

 quartzites beneath the "Salt shales" belong to the Ignacio. 



The Ignacio consists of nearly pure siliceous strata, with some feldspar locally in the 

 lowest beds. The greater part is fine grained, white, gray, or pinkish, and highly indurated. 

 The lower portion is commonly a massive quartzite of prevalent pink or reddish color, while 

 the succeeding strata are nearly white. Distinct bedding is common, as is an irregular jointing. 



