/. D. Dana on American Geological History. 319 



comparatively shallow seas, and at times emerging land ; and 

 was marked out in its great outlines even in the earliest Siluriao. 

 The same view is urged by De Yerneuil, and appears now to be 

 the prevailing opinion among American geologists. The depth 

 at times may have been measured by the thousand feet, but not 

 by miles. 



III. During the first half of the lower Silurian era, the whole 

 east and west were alike in being covered with the sea, In the 

 first or Potsdam Period, the continent was just beneath or at the 

 surface. In the next or Trenton Period, the depth was greater, 

 giving purer waters for abundant marine life. Afterwards, the 

 East and West were in general widely diverse in their forma- 

 tions ; limestones, as Mr. Hall and the Professors Eogers have re- 

 marked, were generally in progress over the West, that is, the 

 region, now the great Mississippi Yalley, beyond the Appalach- 

 ians, while sandstones and shales were as generally forming from 

 northeastern New York south and^ southwest through Yirginia. 

 The former, therefore, has been regarded as an area of deeper 

 waters, the latter as, in general, shallow, when not actually 

 emerged. In fact, the region toward the Atlantic border, after- 

 wards raised into the Appalachians, was already, even before the 

 Lower Silurian era closed, the higher part of the land : it lay as 

 a great reef or sand-bank, partly hemming in a vast continental 

 lagoon, where corals, encrinites and mollusks grew in profusion, 

 thus separating more or less perfectly the already existing At- 

 lantic from the interior waters. 



IY. The oscillations or changes of level over the continent, 

 through the Upper Silurian and Devonian, had some reference 

 to this border region of the continent : the formations approach 

 or recede from it, and sometimes pass it, according to the limits 

 of the oscillations eastward or westward. Along the course of 

 the border itself there were deep subsidences in slow progress, 

 as is shown by the thickness of the beds. It would require 

 much detail to illustrate these points, and I leave them with this 

 bare mention. 



The Hudson Kiver and Champlain valleys appear to have had 

 their incipient origin at the epoch that closed the Lower Silu- 

 rian ; for while the preceding formations cross this region and 

 continue over New England, the rocks of the Niagara and 

 Onondaga Periods (the first two of the Upper Silurian) thin out 

 in New York before reaching the Hudson Eiver. Mr. Logan 

 has recognized the division of America to the northeast into two 

 basins by an anticlinal axis along Lake Champlain, and observes 

 also that the disturbances began as early, at least, as the close of 

 the Lower Silurian, mentioning, too, that there is actually a 

 want of conformity at Graspe between the beds of the Upper 



