GENERAL OBSERVATIONS. 339 



From analogous facts it is learned that the subsidence in the 

 Interior Continental basin was about one mile, instead of seven. 

 In the lower peninsula of Michigan it was at least 2500 feet ; in 

 Illinois, 3000 to 4000 feet ; in Missouri, 5000 to 6000 feet. 



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

 thickness of the Lower Silurian indicates a great subsidence in 

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

 of the great lakes, the 10,000 feet of the Huronian in the last part 

 of the Azoic age, and the 4000 of the Potsdam period, teach that 

 near the beginning of Palaeozoic time this was a region of un- 

 usual 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 Azoic of Canada and New 

 York, the 1000 feet of Trenton limestone, and 700 feet of Calci- 

 ferous and Potsdam beds, prove that there was a great subsiding 

 also in that region, while little occurred south of the New York 

 Azoic. After the Lower Silurian, this subsidence in the vicinity of 

 the Azoic for the most part ceased. 



In Missouri, also, where again there is a patch of Azoic, the 

 thickness of the formations of the Potsdam period was 1500 feet 

 or more. 



All the numbers here given, both for the Appalachian region and 

 the interior, are probably below the actual fact ; for the strata may 

 in many cases — especially along the Appalachian region — have 

 lost much of their original thickness by denudation, either before 

 or after they were consolidated. This loss may have been one- 

 fourth the whole ; but, whatever its extent, it probably has not 

 altered the proportion of subsidence between the Appalachian 

 region and the interior. 



2. Oscillations. — The succession of sandstones, shales, and lime- 

 stones in the Palaeozoic series have been explained to be indica- 

 tions of as many changes in the water-level of the continent. The 

 prevalence of limestones over the Interior basin has pointed out 

 the region as an extensive reef-growing sea, opening south into 

 the Atlantic, and perhaps also the Pacific, for the larger part of 

 Palaeozoic time. But there were slow oscillations in progress that 

 changed the limits of the formations to the eastward or westward, 

 as the periods succeeded one another. 



The Potsdam sandstone covers both the Appalachian region and the Interior 

 basin. 



The Caleiferous beds also stretched over botfi, but were partly limestone in 

 the east, and mostly so in the west. 



