i9o8.] THE PHYSICS OF THE EARTH. 239 



Though Dana's views were somewhat modified by later study 

 and investigation, he always maintained that " the principal moun- 

 tain chains are portions of the earth's crust which have been pushed 

 up and often crumpled or plicated by lateral pressure resulting from 

 the earth's contraction." In order to explain this supposed mode 

 of action he held that the oceanic areas have been " the regions of 

 greatest contraction and subsidence, and that their sides have been 

 pushed like the ends of an arch, against the borders of the con- 

 tinents." 



Even with these arbitrary assumptions it is not at all clear 

 how the settlement of the Pacific Ocean could elevate our great 

 plateau west of the Rocky Mountains, which is nearly a thousand 

 miles wide. If the subsidence of the ocean bed had pushed up the 

 margin of North America, the crumpling and elevation of the land 

 could not well extend one third of the way across the continent. 

 We need not, however, be greatly surprised at this difficulty, for 

 at best Dana's theory is vague, and he evidently could not under- 

 stand just how the elevation had come about. Yet so fully was 

 Dana convinced of the dependence of the mountains on the oceans 

 adjacent to them that he reduced it to calculation by the rule-of- 

 three. He says : 



" The relation of the oceans to the mountain borders is so exact that 

 the rule-of-three form of statement cannot be far from the truth. As the 

 size of the Appalachians to the size of the Atlantic, so is the size of the 

 Rocky chain to the size of the Pacific. Also, as the height of the Rocky 

 chain to the extent of the North Pacific, so are the height and boldness of 

 the Andes to the extent of the South Pacific." (" Manual of Geology," 1863, 

 P- 25.) 



This was indeed a remarkably near approach to the great law 

 of nature, that the mountains along the coasts are formed by the 

 expulsion of lava from under the sea, and are, therefore, every- 

 where proportional to extent and depth of the adjacent oceans. 



§41. Viczi's of James Hall. — In 1857 this distinguished Ameri- 

 can geologist announced in a presidential address to the Ameri- 

 can Association at ]\Iontreal, that the enormous depth of the sedi- 

 mentation along the Appalachian chain was due to the prolonged 

 accumulation of sediments along a sinking, off-shore line of sea 

 bottom. He reached this view from the careful study of the 



