J. D. Dana on American Geological History. 311 



In the outset we are struck with the comparative simplicity 

 of the North American continent, both in form and structure. 

 In outline, it is a triangle, the simplest of mathematical figures ; 

 in surface, it is only a vast plain lying between two mountain / 

 ranges, one on either border, the Appalachian from Labrador to 

 Alabama on the east, the Eocky Mountains on the west ; and on 

 its contour it has water, east, west, north, and south. 



Observe too that its border heights are proportioned to the 

 size of the oceans, A lofty chain borders the Pacific, a low one 

 the narrow Atlantic, while the small Arctic sea is faced by no 

 proper mountain range. 



This principle, that the highest mountains of the continents 

 face the largest oceans, is of wide application, and unlocks many 

 mysteries in physical geography. South America lies between 

 the same oceans as North America : it has its eastern low range, 

 its western Andes ; and as the oceans widen southward, the con- 

 tinent is there pinched up almost to a narrow mountain ridge. 

 It differs from North America in having a large expanse of 

 ocean, the Atlantic, on the north ; and, correspondingly, it has its 

 northern mountain ridges. The world is full of such illustra- 

 tions, but I pass them by. 



This simplicity of ocean boundary, of surface features, and of 

 outline, accounts for the simplicity of geological structure in 

 North America. We may make indeed the wider statement, 

 that all these qualities are some way connected with the posi- 

 tions and extent of the oceans, they seeming to point to the con- 

 clusion, that the subsidence of the oceanic basins had determined 

 the continental features ; and that farther, both results were in- 

 volved in the earth's gradual refrigeration, and consequent con- 

 traction. 



America has thus the simplicity of a single evolved result. 

 Europe, on the contrary, is a world of complexities. It is but t 

 one corner of the Oriental continent,' — which includes Europe, 

 Asia, and Africa, — and while the ocean bounds it on the north 

 and west, continental lands inclose it on the south and east. It 

 has ever been full of cross purposes. American strata often 

 stretch from the Atlantic west beyond the Mississippi ; and east 

 of the Eocky Monntains, it has but one proper mountain range 

 of later date than the Silurian. Europe is much broken up into 

 basins, and has mountains of all ages : even the Alps and Pyre- 

 nees are as recent as the Tertiary. 



This wide contrast accounts for the greater completeness or 

 generality of American revolutions, the more abrupt limits of 

 periods, and clearer exhibition of many geological principles. 



The geological structure of this country has been made known 

 through the combined researches of a large number of investi- 

 gators. The names of Maclure, Silliman, Eaton, lead off the 



