260 ANNUAL OF SCIENTIFIC DISCOVERY. 



up through the Silurian rocks on the east shore of Lake Champlain, 

 and there playing such a conspicuous part, how comes it that a few 

 miles only to the west this formation has entirely disappeared, and the 

 Silurian rocks rest directly upon the formation of sienitic gneiss and 

 limestone which is so largely developed in Northern New York. The 

 same absence is uniformly marked along the whole northern outcrop 

 of this formation, a distance many hundred miles along the northern 

 shore of the St. Lawrence. Again, on the east, we have an Upper 

 Silurian formation, which is traceable from the valley of the Connecti- 

 cut to the Baie des Chaleurs. Along its western base, near the Prov- 

 ince line, we recognize no other than those talcose, micaceous, and 

 magnesian rocks, with their associated deposits, which pass into the 

 so-called Taconic rocks further on by a diminution of the metamor- 

 phism. If these are not the Lower Silurian rocks, it would be neces- 

 sary to suppose that formation entirely absent, for even admitting a 

 want of conformity between them and the Upper Silurian, they should 

 in a region thus disturbed be somewhere exposed. 



To accommodate the theory of the Taconic rocks, it is necessary to 

 suppose in this section a total absence of the Taconic rocks at the 

 western outcrop of the Lower Silurian, and as complete a deficiency 

 of the Lower Silurian on the eastern side of the Taconic. The two 

 great limestone formations mentioned are traced, characterized by 

 their respective fossils, for a distance of 700 miles to the northeast, to 

 the peninsula of Gaspe, with an unvarying breadth between the boun- 

 daries of the two of about 50 miles, constituting a feature not less 

 remarkable geographically than geologically. While, as already re- 

 marked, we have its w r estern border reposing directly upon the sie- 

 nites and crystalline limestones, between these two great formations, 

 both dipping southeast, we have a series which, as they go northeast, 

 gradually lose their metarnorphic character, and are recognized as the 

 rocks of the Hudson River group. Such are the facts that lead to the 

 conclusion, that between the crystalline rocks of the east side of Lake 

 Champlain and the north shore of the St. Lawrence, on the one hand, 

 and the Upper Silurian limestones at the eastern base of the Green 

 Mountain range, on the other, there are no rocks more ancient than the 

 Silurian. 



ON THE DRIFT DEPOSITS OF THE NORTHWEST. 



THE wide-spread drift deposit of the West is particularly conspicu- 

 ous in the rolling prairies. Boulders are sometimes found on its sur- 

 face, generally from ledges far to the north, some of them having 

 been brought 600 miles. As they are as large at their southernmost 

 limit as at the northern, the transporting power can have lost none of 

 its intensity. The drift is the thickest near the Pictured Rocks, where 

 it is 345 feet thick. At Cape Girardeau, above the junction of the 

 Ohio and Mississippi Rivers, it is the thinnest. It extends with scarce- 

 ly an interruption from the Mississippi and the Lakes to the Atlantic. 

 From Zanesville, Ohio, to the Alleghaaies, it is wanting. On the 

 eastern slope of the first branch of these mountains it reappears, and 



