402 On American Geological History. 



After these general remarks on the divisions of Geological 

 time, I now propose to take up the characteristic features and 

 succession of events in American Geology. 



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 

 rano-es, one on either border, the Appalachian from Labrador to 

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

 its contom' it has water, east, west, north and south. 



Observe too that its border heights are proportioned to the 

 size of the oceans. A loftT/ chain borders the Pacific, a loiv one 

 the narrow Atlantic, while the small Arctic sea is foced 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 Am.erica : it has its eastern low range, 

 its western x\ndes ; and as the oceans widen southward, the con- 

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

 It difters 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 positions 

 and extent of the oceans, they seeming to point to the conclusion, 

 that the subsidence of the oceanic basins had determined the 

 continental features ; and that farther, both results were involved 

 in the earth's gradual refrigeration, and consequent contraction. 



also have been so, — 2nd, Graphite is a common mineral in some of the 

 crystalline rocks, aDd graphite is known to result from the alteration by 

 heat of the carbon of plants. — 8rd, the Huronian rocks, according to Sir W, E. 

 Logan, actually contain some small seams of anthracite. — 4th, Vegetation, as 

 it is directly or indirectly the food of animals, should necessarily have 

 preceded animal life. — With reference to the statement in the text above, it 

 should be noted that vegetation has been observed growing among the 

 Geysers of Iceland, in waters having a temperature of 180*^ F. ; and the 

 writer has seen a case of similar kind, on Luzon, one of the Philippines, 

 where the temperature was 160^ F. This is much beyond the limit, which 

 the eggs of animals can endure and survive. 



