384 C. D. Walcott — Position of the Olenellus Fauna. 



Stratigraphic Position of the Olenellus Zone. 



Determined by our present information the Olenellus fauna 

 is at the base of .the Cambrian. Beneath the Olenellus zone 

 the strata are to be referred to some of the j)re-Cambrian groups. 



As already mentioned the Olenellus zone of the Atlantic 

 basin occurs in sediments resting on the Archean and close to 

 the base of the strata referred to the Paleozoic group. In 

 Vermont the Winooski marble series shows over seven hundred 

 feet of limestone beneath the Olenellus zone that have not as 

 yet yielded any characteristic fossils.* Murray and Howley 

 state that in Newfoundland several thousand feet of sandstone 

 occur, on the shores of Trinity Bay and vicinity, beneath the 

 horizon of the Manuel's Brook conglomerate. They do not 

 report fossils, which leaves in doubt the horizon to which this 

 sandstone series should be referred. 



The great series of siliceous rocks of the Wasatch section, 

 IT tali, f show 11,000 feet of strata conformably subjacent to the 

 Olenellus zone ; and all through the uplifts of Cambrian rocks 

 in Utah and Nevada a considerable thickness of strata is known 

 to occur in a similar position. 



In western Nevada the sandstone and siliceous shales of the 

 Wasatch and similar sections are represented by more or less 

 calcareous strata, and it is there that we may hope to find a 

 pre-Olenellus fauna. 



At present I draw the basal line of the Cambrian in Utah 

 and Nevada, at the bottom of the band of arenaceous shale 

 carrying the Olenellus fauna. This refers the quartzites and 

 siliceous shales of the Wasatch and similar sections, including 

 that of the Eureka district and that of the Highland range of 

 Nevada, to the Algonkian Period. 



The section laid bare in the Grand Canon of the Colorado, 

 beneath the great unconformity at the base of the known 

 Cambrian, shows 12,000 feet of unaltered sandstone, shales 

 and limestone that, I think, were deposited in pre-Cambrian 

 time and should be referred to the Keweenawan Group4 This 

 presents one of the best opportunities known to me for the 

 discovery of a pre-Olenellus fauna. The entire section is 

 unbroken, and the sandstones, shales and limestones are much 

 like those of the Silurian section of New York. In a bed of 

 dark argillaceous shale, 3,550 feet from the summit of the 

 section, I found a small Patelloid or Discinoid shell, a fragment 

 of what appears to be the pleural lobe of a segment of a trilo- 

 bite, and in a layer of bituminous limestone, an obscure, small 



* Bull. U. S. Geol. Survey, No. 30, p. 15, par. 13, Nos. 1, 2 and 3 of the section. 



f Loc. cit., p. 38, par. 74. 



\ This Journal, III, vol. xxxii, 1886, p. 153, foot-note. 



