﻿154 C. D. Walcott — Cambrian System of North America. 



sentative of the 13,000 feet of limestone of the same formations 

 in Nevada, and no unconformity, by any extensive erosion, is 

 indicated ; and, again, the 9,000 feet of limestone of the Upper 

 Cambrian and Lower Silurian (Ordovician) of the Central 

 Nevada section is unrepresented in the Wasatch section of 

 Utah. These facts readily prepare us to believe that the hiatus 

 between the Keweenaw and Upper Cambrian is fully equiva- 

 lent to the period of the Lower and Middle Cambrian. 



Another reason is that from the extended orographic move- 

 ment preceding the erosion of the Cambrian, we should expect 

 to find evidence of that erosion in the Cambrian of Utah and 

 Nevada, but, as yet, none such is known. 



Thus far the question of the existence of the Keweenaw sys- 

 tem has been treated from a purely structural basis,* but, in. 

 the course of my study of the distribution of the Cambrian 

 faunas, I have met with some facts that require an explanation 

 and the most plausible one demands the existence of an ex- 

 tended orographic movement, prior to the deposition of the 

 Cambrian strata of the western side of the Continent, that raised 

 a land area over the central portion of the Continent which 

 existed up to the period of the beginning of the deposition of 

 the Upper Cambrian formations, when it was depressed beneath 

 the level of the sea and the Upper Cambrian strata deposited 

 over portions of it. 



The facts demanding explanation are: 1st. The entire ab- 

 sence, as far as known to date, of the Lower Cambrian or Para- 

 doxides fauna west of the Atlantic border : 2d. The absence of 

 the Middle Cambrian or Olenellus fauna over areas occupied 

 by the formations of the Keweenaw system. 



If we accept the view that the Keweenaw, Grand Canon, and 

 Llano strata are outcrops of a system of strata of pre-Cambrian 

 age that extended, in connection with the Huronian and Lau- 

 rentian beneath it and projecting up through it, from the great 

 body of Archean land on the north, southward over the area 

 now occupied by the central portions of the Continent, or the 

 Mississippi Yalley, and westward to the area occupied by sedi- 

 ments accumulated on the western side of the Keweenaw sys- 

 tem of strata when the latter formed a land area, then the ex- 

 planation asked is given. The pre-Keweenaw portion of this 

 Keweenaw land must have been extensive as, in the Missouri 

 area at St. Louis and the Ozark Mountains, the Archean ap- 

 pears beneath the Upper Cambrian ; and all the eastern slopes 



* Professor T. C. Chamberlain gives a most excellent summary of the Keweenaw 

 series and its stratigraphic position in vol. i, of the Geology of Wisconsin. In 

 the section, on page 65, it is placed as a distinct system, resting imconformably 

 on the Huronian which, in turn, is separated from the Laurentian by an uncon- 

 formity. The Cambrian is shown above the Keweenaw as a system between it 

 and the Lower Silurian (Ordovician). 



