MOUNTAINS. The number and the altitude of the mountains of the globe are so great that they form almost everywhere prominent objects, and operate to a large extent in modifying the climatic condition of every country in the world. Yet the amount of solid material so raised above the ordinary level of the land is not so much as might be expected. Remembering that elevated plateaus of great extent occur in several regions, and that the general surface of the earth is considerably higher than the sea level, it has been estimated that were the whole dry land reduced to a uniform level. it would form a plain having an elevation of 1800 ft. above the sea. And were these solid materials scattered over the whole surface of the globe, so as to fill up the bed of the ocean, the resulting level would be considerably below the present surface of the sea. inasmuch as the mean height of the dry land most probably does not exceed -Atli of the mean depth of the bed of the ocean.
Mountains, and especially mountain-chains, subserve important uses in the economy of nature, especially in connection with the water *stem of the world. They are at once the great collectors and distributors of water. In the passage of - moisture-charged winds across them, the moisture is precipitated as rain or snow. When mountain-ranges intersect the course of constant winds by thus abstracting the moisture, they produce a moist country on the windward side, and a comparatively dry and arid one on the lee ward. This is exemplified in the Andes, the precipitous western surface of which has a different aspect from the sloping eastern plain; and so also the greater supply of moisture on the southern sides of the Himalayas brings the snow-line 5.000 feet lower than on the northern side. Above a certain height the moisture falls as snow, and a range of snow-clad summits would form a more effectual separation between the plains on either side than would the widest ocean, were it not that transverse valleys are of frequent occurrence, which open up a pass, or way of transit, at a level below the snow-line. But even these would not prevent the range being an impassable barrier, if the temperate regions con tained as lofty mountains as the tropics. Mountain-ranges, however, decrease in height from the equator to the poles in relation to the snow-line.
The numerous that have been made to generalize on the distribution of mountains on the globe have hitherto been almost unsuccessful. In America the moun tains take a general direction more or less parallel to the meridian, and for a distance of 8,280 miles, from Patagonia tone Arctic ocean, form a vast and precipitous range of lofty mountains, which follow the coast-line in South America, and spread somewhat out in North America, presenting everywhere throughout their course a tendency to separate into two or more parallel ridges, and giving to the whole continent the character of a precipitous and lofty western border, gradually lowering into an immense expanse of eastern lowlands. In the old world, on the other hand, there is no single well-defined continuous chain connected, with the coast-line. The principal. ranges are grouped together in a Y-shaped form, the geneml.direction of which is angles to the new world chain. The center of the system in the Himalayas is the highest land in the hemi sphere. From this, one arm radiates in a north-east direction, and terminates in the high land at Behring straits: the other two take a westerly course; the one a little to the north, through the Caucasus, Carpathians, and Alps to the Pyrenees; the other more to the south, through the immense chain of Central African monntains, and terminating at Sierra Leone. Most of the principal secondary ranges have generally a direction more or less at right angles to this great mountain tract.
The inquiry into the origin of mountains is one that has received not a little attention. Geologists have shown that the principal agents in altering the surface of the globe arc denudation, which is always abrading and carrying to a lower level the exposed surfaces, and an internal force which is raising or depressing the existing strata, or bringing unstratified rocks to the surface. Whether the changes are the small and almost imper ceptible alterations now taking place, or those recorded in the mighty mountains and deep valleys everywhere existing, denudation and internal force are the great producing causes. These give us two great classes of mountains.