Mountain

Mountain, a term somewhat loosely applied even by geologists, but perhaps capable of being restricted to two classes of elevated portions of the earth's surface, as opposed to hills, which form a third class. If hills be taken simply as elevations due to denudation - i.e. to the more extensive wearing away of the surrounding land - mountains may be either of accumulation or of elevation. Mountains of accumulation are simply those originally conical heaps of cinders bound together by penetrating dykes of lava which we know as volcanoes (q.v.). Mountains of elevation generally occur in lines - often not continuous, but en echelon or overlapping - which are known as mountain-chains.

These often coincide in direction with continental areas; but even in such cases as the Andes and the Himalayas the bulk of the chain may be of comparatively modern geological date. It is of the essence of a true mountain-chain that its rocks have undergone tilting, folding, or at least actual upheaval. The chain may be the result of one movement or of a series of movements in one age; but more often it is the result of a long succession of movements in various ages. Originating along a line of weakness in the cooling crust of the shrinking globe, it has again and again been crumpled upwards under the strain of compression. The crest of one fold has often been planed off by denudation and subjected to such a depression as to allow other rocks to be deposited across its denuded edges. [Unconpormability.] These rocks may in their turn have been folded, planed and covered, and so on, at long intervals of geological time.