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Mountain chains are seldom isolated, but are usually combined into systems, consisting of several more or less parallel and connected chains, with their intervening valleys—as the Appalachian system, the Alps, and the Andes.

Most mountain chains seem to have been produced by tremendous lateral pressure in portions of the Earth’s crust, causing either long folds, or deep fissures with upturned edges rising into high ridges, the broken strata forming ragged peaks.