On American Geological History. 413 



as it shown by the thickness of the beds. It would require 

 much detail to illustrate these points, and I leave them with this 



bare mention. 



The Hudson Pwiver and Ohamplain valleys appear to have had 

 their incipient origin at the epoch that closed the Lower Silu- 

 rian ; for while the preceding formations cross this region and 

 continue over New England, the rocks of the Niagara and 

 Onondaga Periods (the first two of the Upper Silurian) thin out 

 in New York before reaching the Hudson River. Mr. Logan 

 has recognized the division of America to the northeast into two 

 basins by an anticlinal axis along Lake Champlain, and observes 

 also that the disturbances began as early, at least, as the close 

 of the Lower Silurian, mentioning, too, that there is actually a 

 want of conformity at Gaspe between the beds of the Upper 

 and Lower Silurian,— another proof of the violence that closed 

 the Lower Silurian era."^ 



But let us pass onward in our geological record. 



All the various oscillations that were in slow movement 

 through the Silurian, Devonian, and Carboniferous Ages, and 

 which were increasing, their frequency throughout the last, rais- 



* This Eastern border of the American continent, then in process of for- 

 mation over the present Appalachian region from Labrador and Canada 

 southwestward, lay deeper to the south than to the north. In Canada and 

 the Azoic of Northern New York, there was land out of water, forming its 

 northern limit. From thence it stretched on with its gradually deepening 

 waters, though varying constantly with the oscillations. The thickness of 

 many of the sedimentary beds passing southward from the New York 

 Azoic prove this increasing depth to have been a general fact ; and it is 

 corroborated by a statement made by Professor W. B. Rogers (meeting of 

 American Association in August last at Albany,) that the subcarboniferous 

 sandstones and shales containing but Uttle limestone in Pennsylvania, were 

 replaced by beds of the subcarboniferons limestone which to the south in 

 Virginia reach a great thickness (see note to page 317)— the limestones 

 indicating cleaver and somewhat deeper waters. The early disturbances 

 and uplifts in the northeast near Gaspo and along the Hudson valley albo 

 accord with this view. 



Again, the position of the Azoic dry land in Canada and of the sedimen- 

 tary rocivs south and southwest, shows us that the Continent in those early 

 times received the northern Labrador current,— which would have kept by 

 the shore as now, along the eastern border of this Azoic,— over New 

 BruusvN^ick and Nova Scotia, and that thence its natural course would have 

 been southwest over the Appalachian region, where the sandstones and 

 shales were extensively accmnulated ; and therefore its aid iu making these 

 deposits can scarcely be doubted. 



