300 PALEOZOIC TIME — DEVONIAN AGE. 



of the United States. But we cannot conclude that no later rocks 

 ever existed over these areas ; for extensive strata may have been 

 washed away in the course of subsequent changes. Yet there had 

 been progress in the dry land from the north, southward, along the, 

 region of Ohio and Wisconsin, as in the State of New York, and , 

 also from the Azoic axis of the far north, westward and east- 

 ward : so that a general expansion of the old Azoic had taken place, 

 by additions to all of its borders. South of New York, and over a 

 large part of the continent, the surface was still liable to alternate 

 sinking and rising, and was, therefore, open to new formations. 

 North America was in the main a continental sea, with the extent 

 of land that was permanently dry very limited as compared with 

 the present finished continent. 



In place of the Rocky Mountains and Appalachians, there were 

 only islands, reefs, and shallow waters, to mark their future site ; 

 for Carboniferous strata and others of later age cover the slopes of 

 the Western mountains even over their summit-plains, and a lime- 

 stone of the Carboniferous age has been observed in one place at a 

 height of 13,000 feet above the sea. The fossils of these rocks are 

 the remains of animals that lived in the continental seas of that 

 region in the next age after the Devonian. The Appalachians also 

 contain in their structure rocks of the Devonian and Carboniferous 

 eras. The Green Mountains have been already spoken of as in 

 part dry land ; but, as rocks of the Devonian and Carboniferous 

 seas constitute portions of New England, they could not have had 

 their present elevation, and were probably low. 



It follows, from the limited area of the land and the absence of 

 high mountains, that there were no large rivers at the time. With 

 the close of the Devonian the Hudson River may have existed 

 with nearly its present limits, and in Canada the Ottawa and other 

 streams drained the northern Azoic. Even the St. Lawrence above 

 Montreal may have been a fresh-water stream. 



2. Rocks df marine and not of fresh-water origin. — None of the strata 

 bear evident marks of fresh-water origin. The shale deposits and 

 mud-rocks, which have been described, are such as might have 

 been formed by fresh waters ; but they all contain fossils, few or 

 many, which are of the same genera and often the same species 

 with those of beds obviously marine. Proofs of fresh-water deposi- 

 tions, therefore, still fail us. Their absence may be accounted for on 

 the grounds — (1) that there were no great rivers ; (2) that whatever 

 material was borne to the sea was worked up by the waves and 

 filled with marine life ; and (3) that depositions over the land, ex- 

 cepting perhaps in those parts of the continent that were perma- 



