GENERAL OBSERVATIONS. 391 



In the later Devonian and the Carboniferous, they included the Hud- 

 son and St. Lawrence (p. 287), and probably, during the Carbon- 

 iferous, the Connecticut. But, even to the last, the region of the 

 great streams of the Rocky Mountains was still a part of the interior 

 sea ; the Mississippi had but a part of its length, and this only 

 temporarily, as the country was often submerged. The valley of the 

 Ohio River was in part the region of the interior Carboniferous 

 marshes : as the mountains in which it rises were not yet raised, the 

 river cannot have existed. Moreover, the Cincinnati uplift (p. 212), 

 which stretched southwestward into Kentucky and Tennessee, and 

 may date from the beginning of the Upper Silurian, probably divided 

 the great interior marshes about the upper Ohio region from those of 

 the lower. 



IV. Oscillations of level. — Dislocations of the strata. 



1. General subsidence. — The earliest Silurian beds, in the Appa- 

 lachian region and New York, — the Primordial, — bear abundant 

 proof, in ripple-marks, sun-cracks, and wind-drifts, of their formation 

 near the water-level. Many of the succeeding strata of the Silurian 

 and Devonian periods contain the same evidence, and lead to the same 

 conclusion for each ; and later, in the Carboniferous formation, many 

 layers show in a similar manner that they were spread out by the 

 w r aves, or within their reach. Consequently, when these last layers 

 of the Paleozoic in the Appalachian region were at the ocean's level, 

 the Potsdam beds — though once also at the surface — were about 

 seven miles below (p. 380) ; for this is the thickness of the strata that 

 intervene ; seven miles of subsidence had, therefore, taken place in 

 that region, during the progress of the Paleozoic ages. 



From analogous facts, it is learned that the subsidence in the In- 

 terior Continental basin may not have exceeded one mile. In the 

 lower peninsula of Michigan, measuring it by the thickness of the 

 rocks, it was at least 2,500 feet ; in Illinois, 3,000 to 4,000 feet ; in 

 Missouri, 5,000 to 6,000 feet. 



On the northern border of the Interior basin, near the Archaean, 

 the thickness of the Lower Silurian indicates a great subsidence in 

 that era, which was not afterward continued. Thus, in the vicinity of 

 the Great Lakes, the 10,000 or 20,000 feet of the Huronian in the last 

 part of the Archaean age, and the 4,000 of the early Lower Silurian, 

 teach that, near the beginning of Paleozoic time, this was a region 

 of unusual subsidence ; and the igneous rocks that intersect and inter- 

 laminate the sedimentary strata evidently came up through the 

 fractures that accompanied, or were occasioned by, the subsidence. 



In Western Canada, between the stable Archa3an of Canada and 



