GEOLOGICAL RELATIONS. 1 3 



Cambrian and Silurian. It is in the Cambrian and Silurian forma- 

 tions that the New England rocks show very distinctive peculiarities, and 

 I have therefore distinguished between them upon the map. In the St. 

 Lawrence, Champlain, and Hudson valley we have the following series : 



In the St. Lawrence valley the bottom member is the Lower Potsdam 

 group. Skirting the west base of the Hoosac and Green Mountains, this 

 is a quartzite i lOO feet thick. Near its termination in northern Vermont 

 there succeeds a mixture of red sandstone and mottled sandy dolomites, 

 about 300 feet thick. In New York and northern Vermont two areas of 

 clay slate occur, with a thickness of at least 3000 feet. In Quebec the 

 older slate, with interstratified thin limestones, belongs to this series, the 

 quartzite and sandstone not appearing. There are limestones of this age 

 at Belle Isle, Anse Au Loup, and Bonne bay. This group is character- 

 ized by the presence of OlcncUus. 



The Upper Potsdam is almost uniformly a sandstone, with fossils indi- 

 cating its existence as an ancient sea-beach. Its surface is covered by 

 ripple marks and crustacean tracks. The thickness along the Champlain 

 valley is about 250 feet ; north of the Adirondacks it rises to 300 and 

 700 feet. About 75 miles below Quebec a large area of related rocks 

 has been described by Richardson, having a thickness of 2000 feet. At 

 the remotest localities named above, the thickness of both the Potsdam 

 periods rises to 11 74 and 2020 feet. At Canada bay the mass is 5600 

 feet thick, according to Murray. 



The calciferous sandrock is about 300 feet thick in the Champlain and 

 St. Lawrence valleys. It crops out occasionally on either shore, and is 

 developed in Newfoundland. It is overlaid by over 1000 feet of lime- 

 stones containing a peculiar set of fossils, referred by Logan and Billings 

 to the Upper Calciferous, a formation entirely absent on the main land. 



Next comes an immense thickness of slaty and calcareous rocks, inter- 

 calated in the geological column quite recently, and known as the Quebec 

 group. Much remains to be learned respecting its limits, but it may em- 

 brace most of the limestones, marbles, and talcoid schists of western 

 Massachusetts and Vermont, amounting to 4000 feet or more. They 

 somewhat exceed this thickness adjacent to the province line, in High- 

 gate, Vt., and Phillipsburgh, P. Q. Logan describes similar rocks near 

 Quebec, with related fossils. He has associated with them, upon theo- 



