Page:The American Cyclopædia (1879) Volume XII.djvu/17

 MOUNTAIN" 9 which Etna and Vesuvius may be taken as types, have been built up as an ant hill is raised by matters brought grain by grain from below the surface. Successive overflows of molten rock or lava, and showers of dust and scoriae, the solidified scum of the lava, have heaped up these volcanic cones; while from time to time fissures or ruptures in the mass have allowed the injection of dikes of molten matter, which in cooling have given solidity to the whole. Yolcanic cones are in fact gen- erated in the air by the force of gravity. Vol- canic vents may occur alike beneath the sea, in low plains, or on elevated plateaus, and some- times from the summits of mountains not them- selves volcanic. (See VOLCANO.) But the moun- tains of purely volcanic origin are insignifi- cant when compared with the great systems of mountains which are not volcanic, or in which the presence of volcanic vents is but a secondary fact. These mountains, whether composed of aqueous or of igneous rocks, have had a very different origin from volcanic cones. They are due to erosion, and are the remains of great plateaus, the larger part of which has been removed. They are but fragments of the upper crust of the earth, separated from each other by valleys which represent the absence or the removal of mountain land. The popu- lar conception is that mountain chains are due to the folding and plication of strata ; but care- ful study of their structure shows that these are but accidents of structure, in no way es- sential to the formation of mountains, and sometimes absent. To De Montlosier and to J. P. Lesley we owe our first conceptions of the true nature and origin of mountains and valleys, and to James Hall its further elucida- tion and its illustration by the facts of North American geology. That the crust of the earth is not rigid, but yielding, and subject to movements of depression and elevation, due to a disturbance of its equilibrium, which have in all ages been operating, is evident from the distribution of sedimentary deposits in past geological periods. In addition to these there are other movements which are conceived to be due to the contraction of the earth's nucleus, resulting also in movements of depression and elevation of the surface, and in corrugations of portions of the crust. The result of these is seen in undulations of the stratified rocks, which are sometimes very slight and regular, but at other times both marked and irregular, occasionally giving rise to great overturns, folds, or inversions, and sometimes enclosing a por- tion of the rocks in a great fold until there is an inversion of the pinched-up strata on both sides of the axis of the fold, by which they come to present a fan-like structure when seen in transverse section. In other cases occur breaks or slidings of the strata on one anoth- er, and frequently more or less nearly vertical displacements, or faults, as they are called, by which the strata on one side of a line of frac- ture may be raised several thousand feet above the same strata on the other side. These va- rious disturbances of the strata influence in many ways the eroding agencies of the ele- ments, so that the mountain outlines and the distribution' of mountains and valleys depend upon these accidents, though not the elevation of the mountain plateau. Thus the crest of a fold from which the strata dip in opposite directions, making what is called in stratigra- phy an anticlinal axis, will generally be frac- tured by the strain which this part has suffered, and will then present a line of weakness which becomes a line of erosion. Valleys are thus cut out, and the strata between the adjacent anticlinals, escaping the eroding action, form a synclinal mountain range, the beds in their natural order dipping from the valleys on each side toward the centre of the mountain. Such a condition of things is seen in the anthracite region of Pennsylvania, in the Catskill moun- tains of New York, and in western Vermont. From irregularities in the undulations, from faults, or from the intervention of harder and softer beds, it often happens that the process of erosion is less regular than this. Sometimes an anticlinal mountain appears ; at other times an anticlinal mountain is divided, presenting two monoclinal mountains, or, as the result of a great fault in the strata, a single mountain of this kind in which the strata dip to one side. For a further discussion of the various forms of mountain structure, see Lesley's "Manual of Coal and its Topography." The structure of mountains is best studied in regions of un- crystalline rocks, where the strata have not been too much disturbed, and where stratifi- cation is very evident, as in the palaeozoic rocks of the Appalachians. In the crystalline eozoic rocks of this mountain system, where the strata are greatly disturbed and nearly vertical, the study of mountain structure is much more diffi- cult. Mountains do not owe their elevation to any folding, or crushing, or piling up of the strata. The influence of folding has been well pointed out by Hall, who has shown the rela- tions of the elevations of palaeozoic rocks in the United States to the accumulation of sediments. In the upper part of the Mississippi valley, where the palaeozoic rocks are represented by 3,000 or 4,000 ft. of sediments, we find hills made up of horizontal strata, the lower Cam- brian rocks which form the base of the hills being everywhere above the water level, while the height of the hills is equal to the vertical thickness of the strata which compose them. In Pennsylvania, on the contrary, where the palaeozoic strata have a vertical thickness of about 40,000 ft., the synclinal mountains, hav- ing in their summits the upper beds of the series, are not more than 2,000 or 3,000 ft. high, the greater part of the strata having been removed from the anticlinal valleys while they are sunk far beneath the mountains. It fol- lows from what has been said that in horizon- tal and synclinal mountains the newer rocks are at the top and the older ones at the base,