﻿152 C. D. Walcott — Garnbrian System of North America. 



ence of the greater portion of the Lower Cambrian (Paradox- 

 ides) fauna, a barrier existed that prevented its extension west- 

 ward of the line mentioned ; that towards the close of the time 

 of the Paradoxides fauna that barrier was removed to the 

 northeast, in the vicinity of Newfoundland, and the descend- 

 ants from the Paradoxides fauna entered the westward seas and 

 spread to the eastern and western basins and formed the Middle 

 Cambrian fauna. What route was taken by the Middle Cam- 

 brian fauna after passing to the western side of the outer 

 barrier is not yet traced, but I think from the indications we 

 now have of a continental area, during Lower and Middle 

 Cambrian time, in the central portion of the continent, that the 

 fauna passed to the south around the southern end of the then 

 existing land, and thence north along the west shore. In the 

 Atlantic basin, the Paradoxides fauna persisted to a greater or 

 less extent and mingled with the types of the Upper Cambrian 

 fauna as in the Upper Lingula Flags of Wales. 



If this is a correct interpretation of the evidence now known, 

 we may look in vain in the central interior basin for the Para- 

 doxides fauna of the Atlantic basin. 



That there was life in the older Cambrian or pre-Cambrian 

 seas of the central interior basin, there is no doubt, as we have 

 found traces of it in the Grand Canon section of Arizona ; and 

 the development of that fauna which from the stratigraphy is 

 pre-Cambrian, is one of the problems awaiting solution. 



During the Upper Cambrian (Potsdam of America ; Upper 

 Lingula Flags of Wales), the Atlantic and Pacific basins appear 

 to have had free communication with each other, and the 

 faunas now have a facies of the same general character. 



The above views are, to a certain extent, theoretical, but the 

 facts demand an explanation other than that the faunas of the 

 Lower, Middle and Upper Cambrian were contemporaneous 

 but in different geographic areas. That the upper and middle 

 faunas were separated by a great interval, is shown by the sec- 

 tions in Nevada and "Vermont ; and that the middle and lower 

 faunas were not contemporaneous is shown by the biologic 

 evidence and the indirect evidence of the absence of the lower 

 fauna in association with the middle fauna in the Newfound- 

 land area, where they are now found in different strata, but a 

 short distance from each other. 



A diagram illustrating the Cambrian sections of America and 

 Europe would show, in the former, that the sequence of life is 

 divided more sharply into three great groups that, in the 

 latter, are more or less broken up. First : by the nearly entire 

 absence of the middle group, and secondly, by the commingling 

 of the upper and lower groups in the European strata and pos- 

 sibly in the Atlantic border sections of New Brunswick and 



