SUBCARBONIFEROUS PERIOD. 317 



and probably they hardly passed the western boundary of Ohio. 

 The Silurian area stretching southwestward from Cincinnati, men- 

 tioned on a former page (p. 228), may have been a barrier between 

 the eastern and western waters. The absence of the limestone 

 from that region and from among the Subcarboniferous strata in 

 eastern Ohio (where it seems replaced by sandstone) affords strong 

 evidence that the elevation about Cincinnati had previously taken 

 place. To the north in Michigan and south in Tennessee, the 

 earlier beds were shales and sandstones, — the former State being 

 near the northern border, and the latter in proximity to the Appa- 

 lachian region. In Michigan the strata are Saliferous, and the con- 

 ditions of the Salina period of the Upper Silurian in New York 

 (p. 249) were probably there repeated. Over both of these districts 

 circumstances were afterwards changed — probably through a pro- 

 gressing subsidence — from those favoring sedimentary accumula- 

 tions to those characterizing the clear Crinoidal sea. But at the 

 same time that sinking was going on in these parts, a rising of 

 the sea-bottom appears to have taken place to the northwestward 

 in Iowa, as Hall has observed ; since after the formation of the 

 Burlington limestone, the succeeding limestone in the series 

 has its northern limit 200 miles more to the southward, and the 

 others still farther south, indicating a contraction of the sea from 

 that direction. The greatest thickness of these limestones, more- 

 over, is in Missouri, where it amounts to 900 feet, or, with the inter- 

 calated arenaceous beds, to 1200 feet. 



It may be again repeated, that no great depth of water was re- 

 quired for the Crinoidal sea. 



The Appalachian region, as in past time, was one of extensive ac- 

 cumulations of conglomerates, sandstones, and shales. Nearly 6000 

 feet of rock — five times the greatest amount in the West — were 

 formed in the course of the period. The thickness of the forma- 

 tions, both in the East and West, is an approximate measure of the 

 amount of subsidence in each. In its more southern part (from 

 Virginia southward), however, there were limestones in progress as 

 in the interior sea. 



The Eastern border region is a repetition in many points, though 

 on a smaller scale, of the more western portion of the continent. 

 The resemblance of the fossils to the European, according to Dawson, 

 implies a more direct connection through the Atlantic with the 

 eastern continent than existed between Europe and the Interior 

 basin. 



