GENERAL OBSERVATIONS. 389 



III. American Geography. 



1. General course of progress. — Through the Paleozoic ages, the 

 dry land of the closing Archaean age (map on p. 149) gradually ex- 

 tended southeastward, southward, and southwestward. At the end of 

 the Silurian, the limit of the dry land appears to have crossed New 

 York, near the central east-and-west line of the State ; and, at the 

 close of the Devonian, it lay not far from its southern border. 

 Westward, beyond Michigan, in Illinois, Iowa, and Minnesota, there 

 was a like expansion to the south and west of the Wisconsin Archaean. 

 Michigan long continued to be a part of the oscillating Interior basin, 

 the Paleozoic formations being continued there, even to the close of 

 the Coal period. 



Along the St. Lawrence, the Ottawa basin was nearly obliterated 

 at the close of the Lower Silurian (p. 216). At the same time, the 

 folding and crystallization of the rocks of the Green Mountains, — the 

 northern portion of the Appalachian chain, — took place ; and the 

 region of the mountains became dry land, and part of the terra fir ma 

 of North America. In the latter half of the Upper Silurian, the 

 river opened into a St. Lawrence gulf over the site of Montreal, and 

 a Lower Helderberg limestone was formed in its waters, upon the up- 

 turned Lower Silurian. The same waters extended southward along 

 Lake Champlain and the Hudson River valley ; and in them Lower 

 Helderberg limestones were formed, on both sides of the Hudson 

 River. In the Devonian age, the head of the St. Lawrence gulf was 

 probably in the vicinity of Quebec, and opened southward over central 

 New England ; for coral reefs were growing in the region of Lake 

 Memphremagog, and in the Connecticut valley, at Littleton, N. H., 

 during the earlier Devonian (p. 256) ; and Crinoids in the same valley, 

 in Massachusetts (p. 237), in the Lower or Upper Helderberg era. 



Still farther south, over part of Rhode Island, lay the Carbon- 

 iferous marshes or coal-making area of the New England basin ; while, 

 to the northeast, over part of Nova Scotia and New Brunswick, the 

 region of the St. Lawrence Gulf, and bordering portions of New- 

 foundland, there were the far larger marshes of the Acadian basin. 

 The two belong geographically to the same great region ■ — then low 

 — between the St. Lawrence and the ocean, but were probably in 

 part separated by the Archaean rocks of northeastern Massachusetts. 



At .the same time, over the rest of the continent, the dry land 

 had expanded nearly to its present extent, and became covered with 

 forests, jungles, and marshes of Carboniferous vegetation. This con- 

 dition oscillated with that of marine submergence, many times in the 

 progress of the Coal period. But the dry land appears to have reached 



