MOUNTAINS. A high elevation of land is called a mountain; a low elevation, a hill. The application of the term °mountain° varies in different localities. In the low lands, where the elevations are not numerous, and do not reach a great height, a rise of ground of about 100 or 200 feet is called a mountain; while in a mountainous country an elevation of 1,000 feet, or less than 2,000 feet, is often called a hill. Prominent parts of mountains or isolated moun tains are called peaks. Some of the common names applied to the parts of a peak, or moun tain, are summit, base, slope, crest, pinnacle, needle, knob. A series of connected mountains form a ridge, range or chain; if the mountains extend a considerable length, the whole is called a range or chain; if short, a ridge. A group of parallel ridges or ranges, or of ridges or ranges near together, usually resultants from similar causes, is called a mountain sys tem; and several systems combined form a cordillera. An elevation with a level-topped area of considerable extent is a plateau and is usually associated with mountains. Many of the highest. mountains of the world rise thousands of feet above the basal plat form of the plateau. The mesa is a large level topped section of the plateau; the butte is a small section. Valleys, lower land than plateaus, exist between the ridges, ranges or systems. A wide valley is often an interior basin. The interior basin area of Australia is over 51 per cent of the whole area; of Africa, 31 per cent; of North America, 32 per cent, and of Eurasia, 28 per cent. (See GREAT BASIN; VALLEYS). Low, narrow places in mountain ridges and chains are called passes. The depressions among the mountains contrib ute in a great measure to a solution of the causes which have determined the kind and lo cation of the vast elevated land masses of the earth.
Mountains have been variously classified, hut the following outline embraces most of the important classes which have been described.
1. Folded Mountains.— These make up the
great mountain systems of the world, and usually consist of very thick seditnentary rocks which by lateral compression have been thrown into more or less closely compressed series of folds or flexures. When first folded, the up arches or anticlines constitute mountains, the troughs or synclines form valleys. Such a situation is found in the Jura Mountains, in Switzerland. In other cases the folds may have been completely worn away to a plain and uplifted. Erosion has then etched out the softer rocks, leaving the edges of the harder rock layers projecting. In such cases the ridges may be the syncline, the flanks of the fold, or the anticline, wherever the hard layer of rock may happen to be. This latter is the history of the Appalachian Mountains. Mountains are frequently regions of intense igneous activity, and the cores of ranges are often occupied by enormous bodies of granitic rock which have melted their way into the sediments, as in the case of the large batholith core of the Sierra Nevada Mountains, in California. The cause of the compression which produces such folds is by no means clear. By some it is attributed to the fact that the supposed molten interior of the earth cools and contracts and that as a re sult the crust, in accommodating itself to the smaller nucleus is forced to crumple. Others believe that great wedge shaned blocks of the earth's c:ust settle toward the centre, and so crowd one another that crumpling is produced at the borders of the blocks. Whatever may be the cause, the fact remains that some enormous force causes lateral thrust, throwing the rocks into great folds and producing thus all the great mountain systems of the world. See FOLDS.
2. Fault Mountains.— Sometimes great breaks occur in the earth's crust, one side of the fracture dropping hundreds of feet, and leaving the elevated block as a mountain. Many of the ranges of western Utah and Nevada are of this origin. See FAmrs.