MOUNTAIN (OP. montaigne, muntaigne, Fr. ;non from AI L. monk:ilea, mwtluna, moun tain. from Lat. mountainous, from mount). A term somewhat loosely used for a high prominence on the earth's surface with steep slopes. The distinction between unnin lain and hill is merely one of size. A plateau is distinguished front a mountain by its broader top and sometimes by its gentler slopes. Where a mountain forms a linear elevation it is ealled a range, ridge, chain, or sierra. Where the ridges are grouped in parallel lines, or en c'ehelon, the groups are called mountain chains, systems, or cordilleras. These terms are sometimes given more precise meanings. Mountain ridge is some times eontined to a series of parallel ranges, all of which were formed within a single geosyncline. or on its borders. Mountain system, is applied to a number of parallel or consecutive ranges formed in separate geosynelines of ap proximately similar dates of upheaval. .1/ounfoin, chitin is applied to two or more systems of the same general region of elevation, but of different dates of origin. Cordinerg is applied to several mountain ebains in the same part of the conti nent. Most of the names applied to parts of mountains explain themselves. Crest, peak. knob, pinnacle. needle, valley are familiar to all. A gorge or cation or clove is distinguished from a valley by its steep slopes and narrow bottom; a park is a. broad valley; a eel or Saddle is an exceptionally low point in a crest ; a cirque is a steeply walled. outward facing amphitheatre near the crest of the range.
Tref:8 ANn oandiN OF MOUNTAINS. Moa11taills are formed (1) by uplift, (2) by eircumdemula thin or land sculpture, and (3) by eruptive agencies; and mountains formed in these three ways have characteristic and distinguishing forms.
(1) By far the larger number of mountains due to uplift in the form of folds or wrinkles in the earth's These folds take on a. great variety of form and arrangement. L'snally number are parallel, but not enextensive in their longer directions, lapping by one another rn i'elielon. They may be upright or overturned at various angles. They may he compressed along the limbs and spread out at the crest in such a way that the strata on both sides of the axial plane of the fold dip toward this plane, a type of folding called the fun fold. sometimes the Alpine structure, because of its excellent develop ment in the Alps. On the major folds with this variety of attitude may be imposed smaller folds, just as smaller waves are superimposed on the greater waves of a body of water. Again, the folds are themselves folded in directions trans verse to their longer directions. Examples of
mountain systems representing folds of the earth's crust are the Alps, the .lura, the Appa lachian system, the Coast Ranges, and most of the Rocky Mountains.
Another type of uplift is shown in the Basin Ranges of Nevada, Idaho, Arizona, and New Mexico, and is known as the Busin Range struc ture, Here the uplift is not accompanied by any horizontal compression. The rocks apparently have been uplifted vertically in great blocks owing their outlines to joints or faults, while others remain stationary or go down. Not infre quently a series of such blocks is tilted in a monoclinal manner, corresponding edges of differ ent blocks going up and the opposite edges going down, the elevated edges thus making mountains, and the depressed edges valleys.
Still another type of uplift is shown in the Uinta Mountains of Utah, where a great mass of rock has been apparently lifted vertically above the adjacent rocks with only a gentle arching.
Two theories have been proposed to explain mountains of the fold type, the so-called gravita tion theory, and the contraction theory. It has been observed that mountains formed by fold ing or faulting are for the most part composed of great thicknesses of sediments which were de posited in the same general geosyncline, i.e, in the same great depression of the earth, or per haps which were deposited in areas which be came geosynelines due to subsidence of the area under the weight of the sediments. It is supposed that the load of sediments may cause the isogeo therms to rise, that is, causing the rocks at a certain level to have a higher temperature than they otherwise would have. 'this, perhaps com bined with the injection of igneous rocks, which are sometimes observed to occur in such areas, causes the rocks to expand. resulting in folds o• mountains. To this there are a number of ob jeetions. The expansion possible by this cause is not sufficient to explain the amount of folding observed, and furthermore it does not explain the occurrence of the folds in parallel ridges. The contraction theory is briefly this: The cooling of the earth is proceeding more rapidly in the interior than at the cool exterior. This causes more rapid contraction of the interior than of the exterior, and the crust, in its tendency to make itself smaller in order to fit the smaller nucleus, becomes wrinkled and corrugated. The v.rinkles become located tilting points of weak ness. These are likely to be under the geosyn clines where great sedimentation has occurred, where, as above seen, the temperature is sup posed to he higher.