﻿C. D. Walcott — Cambrian System of North America. 145 



The Grand Canon section is typical and includes with it the 

 Cambrian section of Central Texas and Northern Wisconsin 

 (see figures 5, 6). 



Crossing to the eastern side of the Continent, our next sec- 

 tion (fig. 7, p. 148) of the Cambrian strata is taken in North- 

 western Vermont, and its contained faunas serve to connect the 

 distant Nevada sections and the group of Cambrian sections 

 along the St. Lawrence, Champlain, and Hudson Eiver Valleys. 



At the base of the section a massive belt of limestone, 1,000 

 feet in thickness, carries in its upper portions the Olenellus 

 fauna which, in the argillaceous shales capping the limestones, 

 attains an extensive development. Continuing up in the sec- 

 tion through the argillaceous shales, about 2,000 feet, masses 



Fig. 5. — Section in Llano County, Texas, showing: the relations of the Upper 

 Cambrian (Potsdam) and the pre-Cambrian Llano Series. 



of limestones are found interbedded in the shales, and in the 

 limestone fossils that show the near approach of the Upper 

 Cambrian or Potsdam fauna. The section gives the same suc- 

 cession of fauna as the sections of Nevada, where we find posi- 

 tive stratigraphic proof of the great difference in age of the 

 Middle and Upper Cambrian faunas. 



The Georgia, Vermont, section includes, in its vertical range, 

 the sections about and below Troy, N. Y., in the Hudson Eiver 

 Valley, and those of Northwestern Newfoundland and the 

 Straits of Belle Isle. 



Directly east of the Adirondack Mountains of New York, 

 the Potsdam sandstone is overlaid by a stratum of shaly 

 arenaceous rock full of fucoidal, or annelid markings, and there 

 the Chazy limestones appear resting on the latter.* Tracing 

 the sandstones south, a fine exposure is seen at Ausable Chasm, 

 and continuing south a limestone is found coming in on top of 

 the sandstone that, in Saratoga County, contains a well-marked 

 fauna of twelve species, four of which are identical with species 

 in the upper beds of the Wisconsin Potsdam sandstone. The 

 calcareous layers of the Potsdam also occur at Whitehall, and 

 Professor Dwight has found them near Poughkeepsie. 



* The unconformity, by non-deposition, noticed by Sir William Logan, is 

 nowhere better illustrated than at this point, the Calciferous formation being 

 absent from the section. 



