172 PALEOZOIC TIME — LOWER SILURIAN. 



I. Rocks : kinds and distribution. 

 1. American. 



The Potsdam formation (numbered 2 on the map, p. 170) ap- 

 pears at the surface just south of and adjoining the Azoic areas in 

 New York and the West ; the strata here exposed are the outcrop- 

 ping portions of a formation that probably underlies all the fossili- 

 ferous rocks of the great Mississippi valley, and stretches south 

 beyond Texas, and north, either side of the Azoic axis of the con- 

 tinent, into the Arctic regions. These Primordial strata have been 

 observed and studied in Texas, about the Black Hills of Dakota 

 and the Laramie Range of the Rocky Mountains, and north in British 

 America. They are also surface-rocks at various places along the 

 Appalachian chain from the St. Lawrence through western Ver- 

 mont to Alabama and Tennessee. East of this chain, they occur 

 near Boston at Braintree, in Newfoundland, and in Labrador. 

 These strata have hence a wide continental range, a universality 

 of distribution unexceeded among later formations. 



The rock of the Potsdam epoch is mostly a laminated sandstone 

 in New York. Along the northern border of the United States, and 

 south, over the Mississippi basin, there is, in addition, some lime- 

 stone ; and a vast thickness of slate or shale exists along the Appa- 

 lachians from the St. Lawrence southwestward. 



The Calciferous epoch, as the name implies, was characterized to 

 a considerable extent by limestone-making, though less strikingly 

 so than the epochs of the Trenton period, which next succeed. 

 The rock is a hard sandstone, more or less calcareous, in New York ; 

 a magnesian limestone, with some sandstone, in the Mississippi 

 basin, where it is often called the Lower Magnesian limestone; sand- 

 stone with very thick shales, and some beds of limestone, along the 

 course of the Appalachians, north and south. 



The thickness of the beds of the Potsdam period in New York, 

 Western Canada, and the Mississippi basin, varies usually from 30 

 to 600 feet ; but along the Appalachians the rocks have an enor- 

 mous development, being from 2000 to 7000 feet thick. 



The rocks of the Potsdam period in many places overlie horizon- 

 tally or nearly so the crystalline Azoic, as illustrated in figs. 136, 

 137, 138. In 136 the Potsdam sandstone (2 a) and Calciferous 

 sandrock (2 b) rest upon the folded Azoic of Essex co., N. Y. ; and 

 in 138, from Canada, there is the same condition, with also the 

 Trenton limestone (3) and Utica shale (4 a) overlying the Potsdam 

 strata. The following cut, from a sketch by J. D. Whitney, represents 



