PLAINS. All those parts of the dry land which cannot properly be called mountainous are plains, and such compose by far the greater part of the earth's surface. Thus, for instance, it has been estimated that in South America the plains are to the mountainous country as 4 to 1. We are not aware that a similar calculation has been made for the other parts of the world, nor are there perhaps materials sufficiently exact for the purpose.
The word plain has but an Indefinite meaning of itself, and seems to be rightly understood only when used in opposition to the word mountains, or when conjoined to the name of some known place, in which case it means the country itself so designated, or the environs of some particular spot. Thus we speak of the cities of the plains, the valleys of the plains, the plains of Lombardy, the plains of Quito, &c.
It were a great error to imagine that by the word plain a perfectly horizontal surface is always understood. In its usual acceptation it means a greater or leas extent of country, flat in its general level as compared with a mountainous country. The more perfectly even and horizontal the surface, the better does it deserve to be called a plain, such as the plains of Venezuela and of the lower Orinoco, Meaopo tamist, itc. But the surface of the ground may be gently waving, as Salisbury plain and the Ukraine ; or more prominently undulated, as the plain round Perla; or it may be studded with Lille, as the plains of the Camicpdare; or it may be traversed by valleys more or less wide and deep, like that part of France which lies between the Loire and the Garonne; or intersect"' with deep ravines, as the central plains of Russia, without ceasing on such accounts to be a plain.
Plains have been divided into two classes, high and low ; but a moment's reflection will show that such denominations can apply rigorously only to the two extremities of a scale of elevation, at the bottom of which would stand, for example, the delta of Egypt or the 'lanes of South America (which latter are raised only about 150 feet above the level of the ocean, and in some places even less), and at the top the plain of Antisana, 13,435 feet abuve the sea-level ; whereas the greater number of plains are found at intermediate heights, as the following will show Feet above the Ocean.
e Though we generally regard those plains which are the least raised above the surface of the ocean as the lowest, it mum, not be forgotten that round the Caspian and Aral them aro plains of many thousand square miles considerably depressed below the sea-level ; as is also the case with the plain or valley of the Jordan.
The term plateau has often been given exclusively to elevated plains; but this also is incorrect, inasmuch as by a plateau is sometimes meant u great extent of country considerably raised above the rest of the land, and having its mountains, its plains, and its valleys, as is particularly exemplified in the minor plateau of Albania, and in the groat plateau of Central Asia, described below.
Table-land, properly so called, is an elevated plain rising more or less abruptly from the general level of the country, and being, as it were, the broad and horizontal or gently undulating top of an immense mountain, as the Nilgherry district of India. Sometimes them are several such, set one upon the other, at least on one or two sides, when they are called platforms or terraces, as those on the eastern slope of the Cordillera of New Mexico.
Some writers regard the words plateau and table-land as merely the French and English names for the same sort of elevation. Humboldt is of opinion that these names should be confined to elevations pro ducing a eensiule diminution of temperature, and, accordingly, to such heights only as attain to 1800 or 2400 feet. Sumo again, as WM, give the name of plateau to all high and extensive mountain-tracts.
Generally speaking, the plains of Europe are of middling elevation, the extremes of high and low being principally found in Asia and America. Thus while the great plains of Central Asia, about Ladak, Tibet, and Katehi, and round Koukounoor and obiewliere, attain a height similar to those of Quito and Titicaca, or from 9000 to 12,000 and 15,000 feet, the great marshy plains of Siberia along the borders of the Frozen Ocean are very slightly raised above the sea-level, as is also the cams with the plains of Bengal at the mouths of the Ganges, the whole of Mesopotamia, the Tolima of Arabia, the.