392 PALEOZOIC TIME. 



New York, the 1,000 feet of Trenton limestone and 700 feet ef Cal- 

 ciferous and Potsdam beds prove that there was a great subsiding 

 also in that region, during the Lower Silurian, while little occurred 

 on the south side of the New York Archaean region. 



In Nova Scotia, the subsidence in the Carboniferous age alone was 

 almost three miles, or nearly half the seven estimated for the Appa- 

 lachians ; and the question of such a subsidence is placed beyond doubt, 

 by finding root-bearing clay-beds and coal-beds at different levels in 

 the series, marking approximately the successive water-levels as the 

 slow subsidence went forward. 



All the numbers here given are probably below the actual fact ; 

 for the strata, in many cases, — especially along the Appalachian re- 

 gion — may 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 successions of sandstones, shales, and lime- 

 stones in the Paleozoic series have been explained to be indications 

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

 lence of limestones over the Interior basin points out the region 

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

 the Mexican Gulf region, and perhaps also into the Pacific, for the 

 larger part of Paleozoic time. But there were slow oscillations in 

 progress, that changed the limits of the formations to the eastward or 

 westward, and northward or southward, as the periods succeeded one 

 another. 



Until the close of the Subcarboniferous period, the oscillations had 

 that wide continental range which was eminently characteristic of the 

 American Paleozoic. In the period following, the Carboniferous, the 

 continent for prolonged periods stood raised just above the ocean, at 

 a nearly uniform level, — so low that its interior was covered with 

 immense fresh-water marshes, and for so long eras that the vegetable 

 accumulations attained the thickness sufficient for great coal-beds 

 (p. 358) ; but these emergences had their alternations with submerg- 

 ences. The system of oscillations, though slower in movement, was 

 still continued ; yet the movements were less general ; and it is there- 

 fore difficult to make out a parallelism in the beds of coal and inter- 

 vening rock-strata through the East and West. 



3. Uplifts and dislocations. — The only mountain-region, along the 

 course of existing chains, which can now be pointed to as having 

 emerged during the Paleozoic ages, is that of the Green Mountains. 



In Nova Scotia, New Brunswick, and parts of the St. Lawrence 



