MOUNTAIN FORMATION. The stresses brought about in the earth's crust through the energy which is causing change in level, whether this be due to contraction, isostasy, or other cause, throw the surface into a series of folds, the largest forming the continental uplifts and ocean depres sions, the smaller forming mountain chains. Ac cording to the contractional hypothesis the gen eral movement of the crust is a downsinking, but locally portions are uplifted because the solid crust cannot accommodate itself to the shrinking interior without wrinkling. The great pressure thus applied to the rocks, operating through long periods of time, causes them to bend or break. Where the rocks which are subjected to these stresses are deeply buried, and hence under great pressure, they bend, even though they are brittle rocks. When the strain is more quickly applied, or when the rocks are nearer the surface, faulting is common; and thick beds of brittle rocks, like sandstone or limestone, are more liable to break than thin-bedded rocks such as shales. See FAULT.
All evidence points to the conclusion that the formation of mountains in the past has been slowly accomplished. Indeed, some mountain chains, such as the Andes and those of the East Indies, the Philippines, and Japan, are now grow ing and apparently little if any more slowly than the mountain growth of the past. It is found
by a study of the structure of mountains that in most cases their growth has been intermittent; that is, periods of freedom from uplift have oc curred. Many of the mountain chains are along lines of crust weakness established in the early periods of geological history. Along these lines the stresses have relieved themselves at various times so that these regions have remained moun tainous throughout geological time. On the other hand, many parts of the crust have been marked by entire freedom from mountain-folding. The zones of mountain growth extend in a general north and south direction in many parts of the earth, as in Western North and South America, Eastern North America, and Eastern Australia, A belt of shorter ranges, with east and west axes, extends across the Old World in the north tem perate zone. Many efforts have been made to find a system in the arrangement of the moun tains of the globe and to account for their dis tribution, but no thoroughly satisfactory theory has been evolved. See CONTINENT; MOUNTAIN, etc.