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.