﻿C. D. Walcott — Cambrian System of Worth America. 139 



strata, not characterized by the presence of fossils, as are strat- 

 igraphically and structurally connected with Cambrian strata 

 identified by organic remains. 



Professor Geikie, in the last edition of his Manual of Geology 

 (1885, p. 65), has included the Cambrian as a subdivision of 

 the Silurian system. I do not now wish to question the wis- 

 dom of this for the geologic section as it occurs in England and 

 the Continent ; but of the presence of a well-defined geologic 

 system beneath the Lower Silurian (Ordovician) strata charac- 

 terized by the Second fauna of Barrande, or the Trenton fauna 

 (including the Upper Calciferous) on the North American con- 

 tinent, there is little doubt. The geologic sections, given in 

 this paper, show that it has a total thickness of over 18,000 

 feet and contains a known fauna of 92 genera, including 393 

 species ; that but very few of these species pass up into the 

 Calciferous horizon of the Lower Silurian (Ordovician), and 

 that the faunas of the two systems are so distinct in their gen- 

 eral facies and also in detail, that they are quite as readily sep- 

 arated as the Lower and Upper Silurian, Silurian and Devon- 

 ian, or Devonian and Carboniferous faunas. There is no doubt 

 that in certain areas the faunas of the Cambrian and Lower 

 Silurian (Ordovician) systems are intermingled in the passage 

 beds between the two systems, but the same is more or less 

 true of all the great divisions of the entire geologic series, from 

 the Archean to the Quaternary. 



Strata of the Cambrian System. 



In beginning the study of the Cambrian system, I looked for 

 well-defined Paleontologic horizons with relation to which the 

 various local sections and their contained faunas could be com- 

 pared. It was evident that the Potsdam faunas of New York 

 and the Mississippi Valley were at or near the summit of the 

 Cambrian, but further than that there was little data. Mr. E. 

 Billings called the Georgia or Olenellus fauna "Lower Pots- 

 dam," and considered the Paradoxides fauna as of older date : 

 but, as late as 1885, one of our best-known paleontologists 

 wrote: "my own impression, at the present, is that the New 

 York typical Potsdam is about equivalent to the lower portion 

 of the Wisconsin areas, and that the Acadian beds of Canada 

 and Vermont, and perhaps the other Atlantic areas, are not 

 appreciably different in age, but that the difference in faunas is 

 more the result of conditions upon which life depended than a 

 difference in time." (Bull. Amer. Mus. Nat. Hist., vol. i, p. 

 140, 1885.) 



The results of the study of the Middle Cambrian faunas will 

 appear in Bulletin 30, of the U. S. Geological Survey, and I 

 have taken much of the data of this paper from the introduc- 



