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Descending into the plains, we find that they are seldom perfectly level, but are formed into slopes of small inclination and of various extent. The pampas of South America, for example, stretch from the base of the Andes to Buenos Ayres, over a space of 900 miles; and in Africa are vast expanses of nearly level land, where the traveller, day after day, sees the horizon preserving the same distance as he proceeds, and bounding an ocean of arid sand. Large flats are also found at great elevations above the sea, such as those of Tartary, Thibet, and Mexico.
Of the other inequalities of the land, the more remarkable are the cavities forming lakes, and the grooves occupied by the beds of rivers. The former are of all sizes, from several hundred miles in circumference down to very small dimensions, and occur in all situations,—between mountain-chains, like the Caspian,—in plains, like Onega,—and along the course of rivers, like those of Canada. The streams necessarily flow in the line which marks the greatest depression of the valleys; although, in some instances, towards their mouths, they occupy a higher level, their beds having been raised by the deposition of the debris carried down by the torrent.
The bottom of the ocean, being merely the continuation of the surface of the land, may be supposed to present inequalities of a similar nature, although, owing to the action of currents, they are probably not so distinctly marked. The transition from what is above to that which is under the water is not in general denoted by any striking phenomenon, excepting[Pg 23] the not unfrequent occurrence of long ranges of cliffs, pebbly beaches, and accumulations of sand. When the coast is low and flat, the depth of the sea in its vicinity is usually small; whilst along a rocky and abrupt shore it generally presents a depression in some measure corresponding to the height of the land. The existence of submarine chains of mountains is established by the numerous shoals and rocks which are to be considered as their summits. On these, coral reefs and islands have been gradually raised by myriads of zoophytes.
The mighty mass of waters, which is collectively termed the sea, occupies, as has been already mentioned, more than two-thirds of the surface of the globe. Its chemical composition, its tides, its currents, and all the varied phenomena which it presents, afford subjects of highly-interesting research.
The atmosphere, in like manner, which envelopes the earth, supplies, in its ever-varying aspects, its motions, its electrical phenomena, and the influence which it exercises on animal and vegetable life, an object of investigation pregnant with curious and useful knowledge.
The mysterious agency of subterranean fire has elevated great masses of rocky matter in various parts of the globe. Earthquakes have effected extensive and remarkable changes upon its surface; the waters of the ocean have alternately worn away the shores and eked them out by depositions of sand and mud; the rivers have furrowed the land, and carried the debris of the higher regions to the valleys and plains; while air and moisture have exercised their decomposing influence upon the hardest substances. By the action of these powers the earth[Pg 24] has become a fit receptacle for the varied forms of animal and vegetable existence with which we see it so profusely stored.
The variable distribution of heat has produced a striking effect in modifying the earth's surface. The cold of the polar regions covers them at all seasons with an extensive deposite of snow and ice, the margins of which are periodically dissolved by the increasing warmth of summer, to be repaired during the succeeding winter. The numberless icebergs, originally formed on the land or in its vicinity, floating on the ocean, and drifted by winds and currents, often pass into more genial regions, producing occasional variations of temperature. The elevated ridges of mountains experience a similar degree of cold, and in all climates, even in the torrid zone, are covered towards their summits with perennial snow.
Limited as are our powers of examining the interior of the globe, we yet find in its crust indications of a power which, by operating so as to produce apparent confusion, has effected results highly beneficial to the beings by whom the earth has been peopled. The strata, at first regularly superimposed upon each other, and consisting of those diversified materials which are supplied by the disintegration of pre-existing rocks, have been broken up, and inclined in every possible degree, so as to form those depressions and elevations which we every where observe on the surface. These inequalities have been increased by the protrusion of masses from the more central regions, and the whole has been subjected to the agency of powerful currents of water, by means of which the angular cavities and projections[Pg 25] have been smoothed or filled up. The consideration of these phenomena constitutes a distinct branch of natural science.
The mountains, rocks, and strata, are composed of ingredients which in themselves are worthy of examination, and capable of affording intense interest. The extremely-diversified forms which these substances assume, their various properties, their uses in the economy of nature, and the purposes to which they may be applied by man, render their investigation not less useful than pleasant.
A most extensive and delightful field of observation presents itself to us in the vegetable bodies with which the surface of the land, and even the depths of the ocean, are so profusely furnished. The various regions of the globe are not less characterized by the form and grouping of the plants which have been allotted to them, than by the comparative activity of their vegetating power. The wastes of Europe, covered by ling, heaths, rushes, and sedges, exhibit little change of aspect under the variations of temperature and the revolutions of the year; while the plains of Venezuela, which during the drought are covered with a layer of sand, and present only a few withered palms scattered along the margins of muddy pools, are converted in the rainy season into an ocean of luxuriant vegetation. In the equinoctial regions of the globe, palms, arborescent ferns, and a multitude of magnificent trees, intertwined with flowering lianas hanging in festoons, form themselves into impenetrable forests, whereas the frigid regions of the arctic circle hardly produce plants a foot in height. The solemn and stately pines of the north of Europe have a very different[Pg 26] aspect from the slender-twigged beeches and chestnuts of its temperate regions, or the laurels and fan-palms of its southern shores.
Viewed in relation to their productions, the gelid regions of the globe are not confined to the circumpolar zone, but extend along the summits of the lofty mountains, following the line of perennial snow, which rises from the level of the sea, in Greenland and Spitzbergen, to the height of 14,000 feet in the Andes. These steril tracts nourish only a few species of plants, although the individuals belonging to them are frequently numerous. In the valleys, and on the southern slopes, no sooner has the returning heat of summer melted the snow, than a beautiful carpeting of verdure, diversified by flowers of various tints, spreads over the soil, displaying an astonishing rapidity of development, while the rocks in many places appear covered with cryptogamic plants. Besides mosses, lichens, and other inferior tribes, multitudes of ferns make their appearance. Grasses and creeping dicotyledonous plants are fully matured; and a rich pasturage affords, during the warm season, abundant nourishment to herbivorous animals. Some trees of small size also appear here and there, or even form themselves into thickets and woods. But, in general, the vegetation of these dreary regions, placed on the limits of the habitable earth, is characterized by a paucity of species and a stunted growth.