It is true that the actual forms of the mountain-ranges are due directly to the action of denuding forces, which have sculptured out from the rude rocky masses all the varied outlines of peaks and crags, of ravines and valleys. But it is none the less true that the determining causes which have directed and controlled all this earth-sculpture, are found in the relative positions of hard and soft masses of rock; but these rock-masses have acquired their hardness and consistency, and have assumed their present positions, in obedience to the action of subterranean forces. Hence we see that though the formation of mountain-ranges is proximately due to the denuding forces, which have sculptured the earth's surface, the primary cause for the existence of such mountain-chains must be sought for in the fact that subterranean forces have been at work, folding, crumpling, and hardening the soft sediments, and placing them in such positions that, by the action of denudation, the more indurated portions are left standing as mountain-masses above the general surface. The old notion that mountain-chains are due to a vertical upthrust from below, finds but little support when we come to study with due care the positions of the rock-masses composing the earth's crust. On the contrary, we find that mountain-ranges are usually carved out of the crushed and crumpled edges of strata which have along certain lines been influenced by great mechanical strains, and subjected to more or less induration and chemical alteration. When we compare these folded and contorted portions of the strata with those parts of the same beds which are not so affected, we find the effects produced in the former are not such as would result from an upthrust from below, but from movements by which a tangential strain would be brought about. If we imagine certain lines of weakness to exist in the solid crust of the earth, then any movements in the portions of the crust between these lines of weakness would cause crushing and crumpling of the strata along the latter. [Sidenote: FORMATION OF MOUNTAIN-CHAINS.] Recent investigations of Dana and other authors have thrown much new light upon the question of the mode of formation of mountain-chains, and the relation between the movements by which they are produced and the sudden and violent manifestations of force witnessed in volcanic outbursts. We cannot, perhaps, better illustrate this subject than by giving a sketch of the series of operations to which the great Alpine chains owe their origin.