EAHJLT CAMBRIAN STRATIGRAPHY. 
95 
allies have a range of 4,900 feet. This section and the one at 
Barrel Spring, Nevada, exhibit the greatest known development 
of fossiliferous Lower Cambrian. 
Nevada, Silver Peak, Barrel Spring. — The Lower Cambrian 
limestones, shales, and quartzites in the vicinity of Silver Peak, 
Nevada, appear to have an exposed thickness of 6,250 feet, 1 
without upper and lower limits, and are much like those east 
of Waucoba Springs, California, with the exception of the 
interruption of the middle of the section by a mass of andesite 
750 feet thick. The genus Olenellus and its congeners (the 
Mesonacidse) appear to have a vertical distribution in this 
section of about 5,300 feet. If there is no duplication in either 
this section or the one near Waucoba Springs, California, these 
vertical distributions for the Mesonacidae of 5,300 and 4,900 
feet, respectively, are of great importance and indicate clearly 
that, in the absence of ample diastrophic criteria, there is little 
justification for assigning to the Pre-Cambrian any of our basal 
quartzitic series, no matter how thick they may be. Such 
occurrences can also be interpreted as indicative of the relatively 
quiescent conditions which obtained in the ocean covering the 
southwestern portion of the continent during the time required 
for the general northeasterly advance of the overlapping portion 
of the same body of water. Traces of organic life are conspicuous- 
ly absent from the major portion of the sediments resulting from 
this encroachment, but favourable habitats in the new sea areas 
appear to have shared the biota of the ocean to the southwest. 
Nevada, Highland Range. — The basal quartzite series is suc- 
ceeded in the Highland range of Nevada by the Pioche forma- 
tion which is stated 2 to be 170 feet thick. A limestone in the 
section at Bennet Springs 3 carries a fauna which is to be 
compared directly with the fauna assigned to the Pioche at 
Pioche (See page 120.) As is explained on page 122, 
in the discussion of the Pioche formation, Mr. Walcott has 
called attention to the relative positions in these faunas of the 
Olenellus horizon and the horizon indicated by the other fossils, 
and to the fact that in the early collections from the Big Cotton- 
'Walcott: Smithsonian Misc. Coll., vol. 53, No. 5, 1908, pp. 188-189. 
1 Idem, No. 1, 1908, pp. 11-12. 
8 Walcott: Mon. U. S. Geol. Survey, vol. LI, 1912, p. 189. 
