344 J, D. Dana on the Plan of Development 



oscillations, causing submergences, for these continued long to 

 occur ; but the gain, on the whole, was a gain — a progress ; and 

 the moving ages made the accession a sure and permanent gain 

 as the continent became more stable. 



II. But in the statement that the growth of the continent was 

 to the south, southeast, and southwest, we assert only the most 

 general truth respecting it. The continent has its special features 

 as much as any being of organic growth, and the elimination of 

 these features is to be traced to the same system of forces. The 

 Appalachian range on the east, the Eocky Mountains and the 

 subordinate chains on the west, the lower lands and lakes of 

 the interior, all in systematic relation, are the more marked of 

 these features ; and the vast river systems, with the broad allu- 

 vial flats and terraced plains, the wide spread drift, the denuded 

 heights and channeled slopes and lowlands, are subordinate pe- 

 culiarities of the face of the continent. 



The Appalachian range of heights, as I explained a year since, 

 was commenced in the Silurian age, and even earlier long before 

 a trace of the mountains had appeared.* The force from the 

 southeast, in the dawn of the Palaeozoic era, had made the Ap- 

 palachian region generally shallower than the Mississippi valley 

 beyond. The vast sandstone and shale deposits of the region 

 bear marks in many parts of sea-shore action, while the limestones 

 which were forming cotemporaneously farther west, indicate 

 clearer and somewhat deeper seas ; and the patch of Azoic in 

 northern New York, lying at the northern extremity of part of 

 the range, points to an anterior stage in the same course of his- 

 tory ; so that, in early time, long before there were mountains, 

 the future of the continent, its low centre and high borders, was 

 foreshadowed. We can hardly doubt that the region of the 

 Eocky Mountains was in the same condition, in the main, with 

 that of the Appalachians. Moreover, these borders, or at least the 

 eastern, for ages anterior to the making of the mountains, were 

 subject to vastly greater oscillations than the interior ; for the 

 Silurian and Devonian sandstones that occur along from New 

 York to Alabama are of great thickness, being five times as 

 thick as the limestones and associated deposits of the same age 

 to the west. A limestone bed, moreover, is of itself evidence 

 of comparatively little oscillation of level during its progress. 



We hence learn that in the evolution of the continental germ, 

 after the appearance of the Azoic nucleus, there were two prom- 

 inent lines of development ; one along the Appalachian region, 

 the other along the Eocky Mountain region — one, therefore, par- 

 allel with either ocean. Landward, beyond each of these devel- 

 oping areas, there was a great trough or channel of deeper ocean 

 waters, separating either from the Azoic area. / 



* Address (fee—See this volume, page 319. 



