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luvium or alluvium, continuing even up to the present time.*

Lowlands.

This variety of the earth's surface stands in the strongest contrast with mountain regions, or, in one word, with the highland form in all its modifications. The name lowland we apply to all those broad tracts which do not rise more than four hundred feet above the level of the sea. The absolute elevation is determined from a section drawn vertically from the superior surface to the plane of the sea. Every comparison by numbers of one lowland plain with its more elevated surroundings gives only a relative result, as for instance, in comparing the valleys of one chain of mountains with those of a more lofty chain. Such relative lowlands may lie at a great elevation above the sea, as the vale of Chamouni, for example, at the north foot of Mont Blanc, is 3000 feet above the ocean level. Both conceptions of the word lowland, which is common to elevated plains as well as those at the sea's margin, are entirely different, and should be kept distinct, although they are very often confounded.

In exact correspondence with the historic progress of upheaval is the internal and external aspect of the result. In direct connection with the extent, course, grandeur, succession of oceanic and volcanic forces, and in constructing new geological formations, is the inexhaustible variety of structure, in respect to continuity, degrees of fracture, as well as the more or less rich prodigality of mineral treasure brought to light. The later formations-the masses injected to fill up huge chasms opened by volcanic pressure from below are easily distinguished from the primitive formation. These courses are usually the depositories of minerals, which the great internal heat has apparently sublimated and crystallized, giving us our gold-sand, rock-salt, and the precious metals.

We are to deal here only with the absolute, great, and generally diffused lowlands, in contrast with which the elevated valleys and plains just referred to may be considered as mountain table lands and the rims of plateaus.

We assume, as we did in judging of the two grades of plateaus, an arbitrary standard of measurement, and limit the rise of real lowlands to an altitude of 500 feet above the level of the sea. Great tracts of running plain, rising by so slight a grade as to be almost imperceptible, can be regarded only relatively as lowland, and, in a strict sense, belong to those regions of transition which fall more truly within the domain of highland or plateau. The word plain indicates the opposite of hill or mountain, but has nothing to do with the greater or less degree of absolute clevation, although it is often used as if it had.

The lower limits of lowlands are sharply defined enough. They are the margin of the sea, toward which the slope usually becomes almost imperceptibly small. Often the expression is used, yet not quite fitly, that the lowland extends into the sea for some distance, and is found beneath the surface. Strictly this is the bottom of the sea, and does not fall under consideration in this connection.

Many lowland plains rise so slightly above the sea level, that they are not unfrequently submerged, and, in many cases, owe their existence to repeated overflows. They are the basins of old gulfs, as in the very slightly elevated plains of Caracas, whose whole shore is open to the influences of the great Atlantic current flowing from east to west; or, as in the great Lombardy plain, which slopes at the same almost imperceptible degree toward the Adriatic. There are also some lowlands found in the interior of continents, and these, too, sinking below the level of the sea; but they are altogether exceptional, and only met with in

two or three instances. They are called, by an accommodation of an algebraic term, negative lowlands. To them belong the region around the Caspian and the Aral Seas, and the much smaller tract comprising the Dead Sea, and forming the Jordan valley; besides, there is the Suez steppe, inclosing the bitter lakes between Asia and Africa; and possibly the Beled-el-Jereed, in the western part of the Sahara and the central part of Australia.

To these it might not be incorrect to join those partial lowlands which have been rescued by human efforts from the sea; the marshes, for instance, behind the dikes of Holland, Sleswick, East Friesland, and at the mouths of the Vistula, the Weser, the Nile, the Ganges, and other rivers.

The most extensive lowlands in the world are probably those which embrace Siberia, in Asia, and the Canadian and polar region of North America. Many great tracts, entirely inland, are in those flat districts covered by seawater which was once driven in by great storms, and now lies stagnant, resulting in inapproachable swamps and morasses. Yet, under the equator, there are immense lowlands, as, for instance, in the eastern Sahara, although this region is broken by strips of plateau, and is by no means that uniform lowland plain which it used to be regarded. Northern Australia belongs to the same category, and also those immense plains which reach from the Atlantic so far into the interior of Brazil, along the lower Amazon. By the time, however, that they reach the middle course of that river, they have acquired, though in such imperceptible steps, a considerable degree of elevation, according to Humboldt's barometrical observations, and not reckoning certain limestone hills found there, from 1050 to 1200 feet. The plains of the middle Marañon are, therefore, true

plains, but not absolute lowlands, and not to be identified. with the great flat region at the mouth of the river, and in comparison with the real lowlands of Venezuela, which do not rise over 200 feet above the sea, and genuine plateau, which, level as it is and broad as it is, is far more elevated than the Valdai plateau, in Russia.

Almost all great river mouths are true lowlands-the Egyptian delta, the delta of the Ganges and the Indus, for instance, (the two latter being separated by the very moderate plateau (100 feet) between Delhi and Mooltan;) to these we may add the delta of the Euphrates, the east shore of China, between the Blue and the Yellow Rivers, and Senegambia, between the Senegal and the Gambia. And in America, the same thing occurs in the Mississippi, Orinoco, Amazon, and La Plata, where the immense mass of water which they send to the sea passes through lowlands of very great extent. In the Mississippi they extend from the mouth as far north as the confluence of the Missouri and the Mississippi, where stands St. Louis, not 500 feet above the level of the sea. The prairies west of the lower course of the river rise rapidly, though imperceptibly to the eye, to the high terraces of Kansas, at Council Grove, varying from 1500 to 2000 feet absolute elevation, and then more rapidly toward the west, to mountain plains or plateaus, from 3000 to 6000 feet high. These, of course, lose the distinctive character of lowland.

The mouth of the St. Lawrence is, in some respects, analogous. Lowlands accompany it for a great distance from the sea; at Lake Ontario the elevation is only 232 feet, at Lake Erie only 565 feet. Yet the level tract is narrowed down to a mere border, and does not widen into great lowland plains. The contracted region of low

country along the St. Lawrence is broken up, too, by rocky heights and rib-like ledges, whose absolute height, however, is not to be confounded with the elevation of the plain which they traverse.

In entire contrast are the broad plains of South America, which lie along the course of the Orinoco, La Plata, and Amazon, the so-called pampas and savannas, which extend a great distance into the interior, farther, indeed, than investigators have yet thoroughly prosecuted their researches. In no continent are the distinctions between highland and lowland so sharply drawn as in America. The lowland plains occupy four-fifths of all the country east of the Andes, in South America: only one-fifth is highland; for, notwithstanding the extent of low plateaus and diminutive mountains scattered through these great plains, yet their entire amount is inconsiderable, compared with the immense lowland tracts of that continent. America has fitly been called the region of the greatest depression on the globe, because this is the prevailing characteristic of its whole eastern side, lowlands forming two-thirds of all America, and highlands only one-third.

In Asia, the later hypsometrical observations have shown that the lowlands are by no means so extensive as they were formerly supposed. The highland extends, according to von Middendorf, much farther northeast of the Yenisei, toward the northern limit of Siberia and Tschatschi, than was formerly supposed; and the Siberian plain extending westward to the Ural Mountains is narrowed down from 4,079,970 to 2,233,800 square miles. Yet this lowland comprises, including central Bokhara or Toorkistan, 1,051,200 square miles, and other low Asiatic plains 1,314,000, the enormous area of 4,599,000, or more than

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