388 PALEOZOIC TIME. 



lated ridges in the seas of the Rocky Mountain region, etc. The 

 Carboniferous marshes covered a large part of the site of the Alle- 

 ghanies, and a sea in which Carboniferous limestones were forming, 

 a considerable portion— perhaps all but the Azoic heights — of the 

 area of the Rocky Mountains. 



But after the close of the Lower Silurian the Green Mountain re- 

 gion appears to have been a'bove the sea (pp. 228, 243), and divided 

 the New England or Eastern border region from the Interior. Conse- 

 quently, the subsequent progress of the dry land over New England 

 was from the Green Mountain region eastward, as well as from the 

 St. Lawrence southward. In other words, the Devonian beds which 

 stretch from Gaspe to Vermont, thence bend southward on the east of 

 those mountains, as lias been suggested by the geologists of Canada. 



3. Rivers.-— The rivers of the early Palaeozoic were only small 

 streams, such as might have gathered on the limited Azoic lands. 

 In the later Devonian and the Carboniferous, they included the 

 Hudson and St. Lawrence (p. 300). 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 Carbonife- 

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

 raised, the river cannot have existed. Moreover, the Cincinnati 

 uplift (p„ 228), 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. 



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



1. General subsidence.^ The earliest Silurian beds — the Potsdam — 

 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 evi- 

 dence, 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 waves, or within their reach. Conse- 

 quently, when these last layers of the Palaeozoic in the Appalachian 

 region were at the ocean's level, the Potsdam beds — though once 

 also at the surface— were about seven miles below ; 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 Palaeozoic ages. 



