808 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



east and southwest accumulations, to be found in the northern 

 hemisphere, the whole of the wrinkling consequent on the internal 

 contraction of the earth (from various causes) is concentrated, and 

 this is precisely because the great accumulation of sediments along 

 these lines has softened the rigid upper crust, and thus rendered 

 the folding more easy there than elsewhere. The cause of eleva- 

 tion is to be sought, partly in such foldings, l^ut chiefly in a sub- 

 version of the balance of pressure on the somewhat yielding crust 

 of the earth, consequent upon the transference of sedimentay mate- 

 rial from one area to another. 



Mountains are not due to local uplifts, but to original deposition 

 and sul)sequent continental elevation, modified by erosion, and in 

 most cases (but not necessarily) by the results of undulation. Ero- 

 sion operates by preference along the anticlinals, which are lines 

 of weakness, and hence the oldest rocks appear in the valleys, 

 while the newest are on the mountain tops. Mountains are but 

 fragments of wasted continents, which have been spared in the 

 general erosion. 



Age of Mountains. 



The newest mountains, other things being equal, are the most 

 lofty. The Highlands of the Hudson, the Adirondacks, and the 

 Laurentidcs, which belong to the great Laurentian system of rocks, 

 are the oldest hills known on the globe, and had essentially their 

 present form before the materials of the Green Mountains, which 

 are of a lower silurian age, were spread over the sea bottom. 

 The White Mountains are still newer, being of the same age as the 

 Catskills of New York, and consisting of the same Devonian rocks, 

 which in New Hampshire are crystalline from metamorphism. 



The Catskill and the White Mountains are the remaining sepa- 

 rated portions of an immense Devonian plateau, which was once 

 spread across New England, from Pennsylvania to the St. Law- 

 rence. But even these mountains are old when compared with 

 the great mountain chains of our western coast and of Central 

 Europe and Asia, which, geologically speaking, are but of yester- 

 day. The summit of the Alps are of tertiary sediments, which 

 were being deposited beneath the sea ages after this ancient conti- 

 nent had its pi-esent form and relief. 



• History of the Theory of Mountains. 

 The old European notion of locally uplifted mountains, with 

 granite centres, still finds its place in text-books, but will soon' be 



