98 J. D. Dana on the Origin of Continents. 
mountains of the earth are about equal in comparative altitude 
to the thickness of the cracked varnish on a twelve inch globe. 
We remark, again, that we exclude none of those causes of 
elevation usually recognized, which facts show to have been in 
operation, though allowing them only a subordinate place. 
From these explanations, we proceed to the application of them. 
If the reader will place before him a good map of North Amer- 
ica, he will perceive at once the effects which have been alluded 
to exhibited on a grand scale, on both sides of the continent. On 
the Atlantic side, the Appalachians, from Maine to Georgia, con- 
sist of rock strata, which have been variously folded up into 
ridges, as has been made out with great beauty and fullness by 
Professors W. B. and H. D. Rogers.* These folds are in several 
series, but are nearly uniform or parallel in position. As should 
be expected from the nature of the cause, the plications are more 
frequent and abrupt on the side of the chain nearest the ocean, 
and gradually die out westward just beyond the limits of the 
Appalachians. As another result of proximity to the contract- 
ing area, the rocks on the eastern side have been most altered 
by the very contraction which occasioned the depression ; an 
between lies a vast plain, scarcely affected at all by these changes, 
the great central area of the continent. ‘This view is farther sus- 
* Trans. of the Assoc. of Amer. Geol. and Nat., 1840. ] : , al is 
Journal, xliii, 177; xliv, 359. pare AS ames REN ler 
_ | See the section of the region between the mouth of the Kansas and Fort Van- 
couver, by Captain Fremont, in the Report of his Exploring Expedition to the 
Rocky Mountains in 1842, and to Oregon and North California in 1843, 1844. 
Printed by order of the Senate of the United States, Washington, 1845. 
