Page:The New International Encyclopædia 1st ed. v. 14.djvu/85

MOUNTAIN. 67 along the limbs ami spread out at the crest in such a way that the strata un both sides of the axial plane of the fold dip toward this plane, a tjpe of folding called the fan fold, sometimes the Alpine ntnttluic, 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 Jura, the Appa- lachian system, the Coast Ranges, and most of the Rockv .Moiuitains. MOUNTAIN. SYMMETRICAL FOLD, JURA .MOUNTAI.Ve. Another type of uplift is shown in the Basin Ranges of Nevada, Idaho, Arizona, and New Mexico, and is known as the Basin liutKje 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- FACLTED STRUCTURE, BASIN RANGE. 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 tints 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. BROAD MONOCLINE, UINTA MOUNTAINS. 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 geosynclines due to subsidence of the area under the weight of the sediments. It is supposed that the load of sediments may catise the isogeo- therins 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 or mountains. To this there are a number of ob- jections. The expansion possible by this eau.se is not sullicient 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 brielly 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 wrinkles become located along 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 be higher. (2) 'hen an area is elevated above the sea the natural forces of erosion, wind, water, changes of temperatui-e, etc., begin their attack upon the land, and slowly cut it down, the waters collecting in rills, brooks, and rivers, cut- ting small and large valleys with a variety ot forms and distribution due to the var.ying hard- ness and the structure of the materials cut through. If carried far enough this process will ultimately bring the elevated area to sea level. At certain stages in the process certain areas, because of their hardness or strttcture, may stand high above the surrounding areas which have been worn away, and may be called mountains. In so far a.s mountains have already been formed by folding and elevation, subsequent erosion only modifies their shapes, but where uplift has not left the land in mountain form subsequent de- nudation may bring it to this point. Mountains formed or modified in this way have a variety of shapes. When the strata ai'e horizontal a hard layer at the top may resist erosion sufficiently long to make the mountain a flat or table top mountain. Hard volcanic material about a vol- cano may resist erosion more than the surround- ing material, and the area thus stand as a lava- topped mountain. If the strata are much folded erosion cuts down along lines of least resist- ance, usually following the softer strata, leaving the edges of the harder ones as ridges. This state of affairs appears in the Appalachian system, a system owing its present features to folding and elevation combined with diflerential denudation along softer layers. (.3) Mountains formed by eruption of igneous rocks are of common occurrence, ^'olcanic moun- tains formed by lavas occurring either singly or grouped in lines are well known. Vesuvius, Rainier. Hood. Saint Helen's, and Shasta are ex- amples of these. The volcanic materials may be so grouped as to form an upland of consider- able extent, as the Cascade range. Eruptions of igneous rocks which ne'er reach the surfac,e also form mountains by bulging up the strata above them. Going ttpward toward the surface they find it easier at a certain point to spread out in a globular form (called laccoliie). arch- ing the strata above them, than to break through the overlying .strata and come to the surface. The mountain thus formed is a dome with a nucleus of igneous material. The Henry and Huerfano mountains of the Western United States are of this type. MOFNTAINS AND Ci-IMATE. ^Mountains fulfill important usci^ in the economy of nature, espe- ciall.v in connection with the water system of the world. They are at once the great collectors