North America (continued)—The Great Central Plains, or Valley of the Mississippi—The Alleghany Mountains—The Atlantic Slope—The Atlantic Plain—Geological Notice—The Mean Height of the Continents.

The great central plain of North America, lying between the Rocky and Alleghany Mountains, and reaching from the Gulf of Mexico to the Arctic Ocean, includes the valleys of the Mississippi, St. Lawrence, Nelson, Churchill, and most of those of the Missouri, Mackenzie, and Coppermine rivers. It has an area of 3,245,000 square miles, which is 245,000 square miles more than the central plain of South America, and about half the size of the great plain of the old continent, which is less fertile; for although the whole of America is not more than half the size of the old continent, it contains at least as much productive soil.

The plain, 5000 miles long, becomes wider towards the north, and has no elevations, except a low table-land which crosses it at the line of the Canadian lakes and the sources of the Mississippi, and is nowhere above 1500 feet high, and rarely more than 700: it is the watershed between the streams that go to the Arctic Ocean and those that flow to the Mississippi. The character of the plain is that of perfect uniformity, rising by a gentle regular ascent from the Gulf of Mexico to the sources of the Mississippi, which river is the great feature of the North American low lands. The ground rises in the same equable manner from the right bank of the Mississippi to the foot of the Rocky Mountains, but its ascent from the left bank to the Alleghanies is broken into hill and dale, containing the most fertile territory in the United States. Under so wide a range of latitude the plain embraces a great variety of soil, climate, and productions; but, being almost in a state of nature, it is characterized in its middle and southern parts by interminable grassy savannahs, or prairies, and enormous forests, and in the far north by deserts which rival those of Siberia in dreariness.

In the south a sandy desert, 400 or 500 miles wide, stretches along the base of the Rocky Mountains to the 41st degree of N. lat. The dry plains of Texas and the upper region of the Arkansas have all the characteristics of Asiatic table-lands; more to the north, the bare treeless steppes on the high grounds of the far west are burnt up in summer, and frozen in winter by biting blasts from the Rocky Mountains; but the soil improves towards the Mississippi. At its mouth, indeed, there are marshes which cover 35,000 square miles, bearing a rank vegetation, and its delta is a labyrinth of streams and lakes, with dense brushwood. There are also large tracts of forest and saline ground, especially the Grand Saline between the rivers Arkansas and Neseikelongo, which is often covered two or three inches deep with salt like a fall of snow. All the cultivation on the right bank of the river is along the Gulf of Mexico and in the adjacent provinces, and is entirely tropical, consisting of sugar-cane, cotton, and indigo. The prairies, so characteristic of North America, then begin.

To the right of the Mississippi these savannahs are sometimes rolling, but oftener level, and interminable as the ocean, covered with long rank grass of tender green, blended with flowers chiefly of the liliaceous kind, which fill the air with their fragrance. In the southern districts they are sometimes interspersed with groups of magnolia, tulip, and cotton trees; and in the north with oak and black walnut. These are rare occurrences, as the prairies may be traversed for many days without finding a shrub, except on the banks of the streams, which are beautifully fringed with myrtles, azaleas, kalmias, andromedas, and rhododendrons. On the wide plains the only objects to be seen are countless herds of wild horses, bison, and deer. The country assumes a more severe aspect in higher latitudes. It is still capable of producing rye and barley in the territories of the Assinaiboia Indians, and round Lake Winnepeg there are great forests; a low vegetation with grass follows, and towards the Icy Ocean the land is barren and covered with numerous lakes.

East of the Mississippi there is a magnificent undulating country about 300 miles broad, extending 1000 miles from south to north between that great river and the Alleghany mountains, mostly covered with trees. When America was discovered, one uninterrupted forest spread over the country, from the Gulf of St. Lawrence and the Canadian lakes to the Gulf of Mexico, and from the Atlantic Ocean it crossed the Alleghany mountains, descended into the valley of the Mississippi on the north, but on the south it crossed the main stream of that river altogether, forming an ocean of vegetation of more than 1,000,000 of square miles, of which the greater part still remains. Although forests occupy so much of the country, there are immense prairies on the east side of the river also. Pine barrens, stretching far into the interior, occupy the whole coast of the Mexican Gulf eastward from the Pearl River, through Alabama and a great part of Florida.

These vast monotonous tracts of sand, covered with forests of gigantic pine-trees, are as peculiarly a distinctive feature of the continent of North America as the prairies, and are not confined to this part of the United States; they occur to a great extent in North Carolina, Virginia, and elsewhere. Tennessee and Kentucky, though much cleared, still possess large forests, and the Ohio flows for hundreds of miles among magnificent trees, with an undergrowth of azaleas, rhododendrons, and other beautiful shrubs, matted together by creeping plants. There the American forests appear in all their glory: the gigantic deciduous cypress, and the tall tulip-tree, overtopping the forest by half its height, a variety of noble oaks, black walnuts, American plane, hickory, sugar-maple, and the lireodendron, the most splendid of the magnolia tribe, the pride of the forest.

The Illinois waters a country of prairies ever fresh and green, and five new states are rising round the great lakes, whose territory of 280,000 square miles contains 180,000,000 of acres of land of excellent quality. These states, still mostly covered with wood, lie between the lakes and the Ohio, and they reach from the Ohio river to the Upper Mississippi—a country twice as large as France, and six times the size of England.

The quantity of water in the north-eastern part of the central plain greatly preponderates over that of the land; the five principal lakes, Huron, Superior, Michigan, Erie, and Ontario, cover an area equal to Great Britain [and Ireland], without reckoning small lakes and rivers innumerable.

[The north-west country, or Upper Mississippi valley, comprehends about ten degrees of latitude, from 39° to 49° north, and about fourteen degrees of longitude, from 87° to 101° (from 10° to 24° from the meridian of Washington), and contains about 300,000 square miles. A large part of this tract, consisting of the northern portion, is still held by the Indians.

This country has some very peculiar natural features. The most remarkable of these is the numberless lakes which spangle its northern surface, the remains, no doubt, of a vast sea that once covered the whole country extending north from the Gulf of Mexico, possibly to Hudson’s Bay.

The country, from the outlets of the Illinois and Missouri rivers to St. Peter’s, and from Lake Michigan to Council Bluffs, and beyond that point westerly, is a vast gently-inclined plane, ascending to the north and to the west. Between the Mississippi and the lake the elevation above the Atlantic has been found to be a little more than 500 feet: and west of the river, on the same parallel, towards the Missouri, something more than 700 feet. At St. Peter’s it is about 700 feet. Nicollet states that Council Bluffs is 1037 feet above the gulf; and the elevation of Rock Island, in the same latitude on the Mississippi, he says, is 528; and the height of Fort Pierre Chouteau, on the Missouri, on the same authority, is 1456; the lower end of Lake Pepin, in the same latitude (44° 24' north), is 710 feet, and the mouth of the St. Peter’s, in about latitude 45°, is 744 feet. There are a few elevations above the general range, called mounds; but with the exception of these, the surface is marked only by ravines running down to the beds of the streams, which are usually from one to two hundred feet lower.

There are large tracts of this north-west country wholly destitute of tree or shrub, and covered only with a luxuriant growth of wild grass, and beautifully interspersed with flowers of every hue and variety, each successively making the prairie to look gay with their presence from April to October. This beautiful natural meadow yields bountiful returns for culture and toil bestowed upon it. It consists of a very dark-brown vegetable mould, and is mellow beyond the conception of those who are acquainted only with the hard, stiff soils of the Atlantic slope. This mould is from one and a half to two feet deep, and entirely free from gravel. The sub-soil is yellow light clay or clay loam, which resembles the soil of timbered lands. The country is a limestone formation. Timber is found only along the streams: it consists of elm, ash, black walnut, butternut, maple, mulberry, and iron-wood, on the bottoms; and on the upland, white, red, black, and burr-oaks, shell-bark and common hickory, with, occasionally, linden, birch, wild-plum and cherry, locust, and some other trees. On the Wisconsin and St. Croix rivers are heavy growths of pine, from which supplies of lumber are carried down the Mississippi river.^[61]

The mighty rivers of this region must be measured by travel, the prairies must be crossed, and the lakes seen before the mind fully comprehends a description of them. “To look at a prairie up or down,” says Nicollet, “to ascend one of its undulations; to reach a small plateau (or, as the voyageurs call it, a prairie planchÉ), moving from wave to wave over alternate swells and depressions; and, finally, to reach the vast interminable low prairie that extends itself in front,—be it for hours, days, or weeks, one never tires; pleasurable and exhilarating sensations are all the time felt; ennui is never experienced. Doubtless there are moments when excessive heat, a want of fresh-water, and other privations, remind one that life is toil; but these drawbacks are of short duration. There is almost always a breeze over them. The security that one feels in knowing that there are no concealed dangers—so vast is the extent which the eye takes in—no difficulties of road; a far spreading verdure, relieved by a profusion of variously coloured flowers; the azure of the sky above, or the tempest that can be seen from its beginning to its end; the beautiful modifications of the changing clouds; the curious looming of objects between earth and sky, taxing the ingenuity every moment to rectify:—all, everything, is calculated to excite the perceptions and keep alive the imagination. In the summer season, especially, everything upon the prairies is cheerful, graceful, and animated. The Indians, with herds of deer, antelope, and buffalo, give life and motion to them. It is then they should be visited; and I pity the man whose soul could remain unmoved under such a scene of excitement.”]

The Canadas contain millions of acres of good soil, covered with immense forests. Upper Canada is the most fertile, and in many respects is one of the most valuable of the British colonies in the west: every European grain, and every plant that requires a hot summer and can endure a cold winter, thrives there. The forests consist chiefly of black and white spruce, the Weymouth and other pines—trees which do not admit of undergrowth: they grow to great height, like bare spars, with a tufted crown, casting a deep gloom below. The fall of large trees from age is a common occurrence, and not without danger, as it often causes the destruction of those adjacent; and an ice-storm is awful.

After a heavy fall of snow, succeeded by rain and a partial thaw, a strong frost coats the trees and all their branches with transparent ice often an inch thick; the noblest trees bend under the load, icicles hang from every bough, which come down in showers with the least breath of wind. The hemlock-spruce especially, with its long drooping branches, is then like a solid mass. If the wind freshens, the smaller trees become like corn beaten down by the tempest, while the large ones swing heavily in the breeze. The forest at last gives way under its load, tree comes down after tree with sudden and terrific violence, crushing all before them, till the whole is one wide uproar, heard from afar like successive discharges of artillery. Nothing, however, can be imagined more brilliant and beautiful than the effect of sunshine in a calm day on the frozen boughs, where every particle of the icy crystals sparkles, and nature seems decked in diamonds.^[62]

Although the subsoil is perpetually frozen at the depth of a few feet below the surface beyond the 56th degree of north latitude, yet trees grow in some places up to the 64th parallel. Farther north the gloomy and majestic forests cease, and are succeeded by a bleak, barren waste, which becomes progressively more dreary as it approaches the Arctic Ocean. Four-fifths of it are like the wilds of Siberia in surface and climate, covered many months in the year with deep snow. During the summer it is the resort of herds of rein-deer and bisons, which come from the south to browse on the tender short grass which then springs up along the streams and lakes.

The Alleghany or Appalachian chain, which constitutes the second or subordinate system of North American mountains, separates the great central plain from that which lies along the Atlantic Ocean. Its base is a strip of table-land, from 1000 to 3000 feet high, lying between the sources of the rivers Alabama and Yazoo, in the southern states of the Union, and New Brunswick, at the mouth of the river St. Lawrence. This high land is traversed throughout 1000 miles, between Alabama and Vermont, by from three to five parallel ridges of low mountains, rarely more than 3000 or 4000 feet high, and separated by fertile longitudinal valleys, which occupy more than two-thirds of its breadth of 100 miles. In Virginia and Pennsylvania, the only part of the chain to which the name of the Alleghany mountains properly belongs, it is 150 miles broad, and the whole is computed to have an area of 2,000,000 of square miles. The parallelism of the ridges, and the uniform level of their summits, are the characteristics of this chain, which is lower and less wild than the Rocky Mountains. The uniformity of outline in the southern and middle parts of the chain is very remarkable, and results from their peculiar structure.^[63] These mountains have no central axis, but consist of a series of convex and concave flexures, forming alternate hills and longitudinal valleys, running nearly parallel throughout their length, and cut transversely by the rivers that flow to the Atlantic on one hand, and to the Mississippi on the other. The watershed nearly follows the windings of the coast from the point of Florida to the north-western extremity of the State of Maine.^[64]

The picturesque and peaceful scenery of the Appalachian mountains is well known; they are generally clothed with a luxuriant vegetation, and their western slope is considered one of the finest countries in the United States. To the south they maintain a distance of 200 miles from the Atlantic, but approach close to the coast in the south-eastern part of the State of New York, from whence their general course is northerly to the river St. Lawrence. But the Blue Mountains, which form the most easterly ridge, are continued in the double range of the Green Mountains to GaspÉ Point in the Gulf of St. Lawrence. They fill the Canadas, Maine, New Brunswick, and Nova Scotia with branches as high as the mean elevation of the principal chain, and extend even to the dreary regions of Baffin’s Bay. The chief Canadian branches are parallel to the river St. Lawrence. One goes N.E. from Quebec; and the Mealy Mountains, which are of much greater length, extend from Ottowa River to Sandwich Bay, and, though low, are always covered with snow. Little is known of the high lands within the Arctic Circle, except that they probably extend from S.E. to N.W.

The country between Hudson’s Bay, the mouths of the Churchill and that of the Mackenzie river, is also an unknown region; on the east it descends steeply to the coast, but the western part, known as the Barren Ground, is low and destitute of wood, except on the banks of the streams. The whole is covered with low precipitous hills. Not only the deep forests, but vegetation in general, diminishes as the latitude increases, till on the arctic shores the soil becomes incapable of culture, and the majestic forest is superseded by the arctic birch, which creeps on the ground. Many of the islands along the north-eastern coasts, though little favoured by nature, produce flax and timber: and Newfoundland, as large as England and Wales, maintains a population of 70,000 souls by its fisheries: it is nearer to Britain than any part of America—the distance from the port of St. John to the harbour of Valentia in Ireland is only 1656 nautical miles.

The long and comparatively narrow plain which lies between the Appalachian mountains and the Atlantic extends from the Gulf of Mexico to the eastern coast of Massachusetts. At its southern extremity it joins the plain of the Mississippi, and gradually becomes narrower in its northern course to New England, where it merely includes the coast islands. It is divided throughout its length by a line of cliffs from 200 to 300 feet high, which begins in Alabama and ends on the coast of Massachusetts. This escarpment is the eastern edge of the terrace known as the Atlantic Slope, which rises above the Maritime or Atlantic Plain, and undulates westward to the foot of the Blue Mountains, the most eastern ridge of the Appalachian chain. It is narrow at its extremities in Alabama and New York, but in Virginia and the Carolinas it is 200 miles wide. The surface of the slope is of great uniformity; ridges of hills and long valleys run along it parallel to the mountains, close to which it is 600 feet high. It is rich in soil and cultivation, and has an immense water power in the streams and rivers flowing from the mountains across it, which are precipitated over its rocky edge to the plains on the west. More than twenty-three rivers of considerably size fall in cascades down this ledge between New York and the Mississippi, affording scenes of great beauty.^[65]

Both land and water assume a new aspect on the Atlantic Plain. The rivers, after dashing over the rocky barrier, run in tranquil streams to the ocean, and the plain itself is a monotonous level, not more than 100 feet above the surface of the sea. Along the coast it is scooped into valleys and ravines, with innumerable creeks.

The greater part of the magnificent countries east of the Alleghanies is in a high state of cultivation and commercial prosperity, with natural advantages not surpassed in any country. Nature, however, still maintains her sway in some parts, especially where pine-barrens and swamps prevail. The territory of the United States occupies 7,000,000 or 8,000,000 of square miles, the greater part of which is capable of producing everything that is useful to man, but not more than a twenty-sixth part of it has been cleared. The climate is generally healthy, the soil fertile, abounding in mineral treasures, and it possesses every advantage from navigable rivers and excellent harbours. The outposts of civilization have already advanced half-way to the Pacific, and the tide of white men is continually and irresistibly pressing onwards to the ultimate extinction of the original proprietors of the soil—a melancholy, but not a solitary, instance of the rapid extinction of a whole race.

Crystalline and Silurian rocks, rich in precious and other metals, form the substratum of Mexico, for the most part covered with plutonic and volcanic formations and secondary limestone; yet granite comes to the surface on the coast of Acapulco, and occasionally on the plains and mountains of the table-land. The Rocky Mountains are mostly Silurian, except the eastern ridge, which is of stratified crystalline rocks, amygdaloid and ancient volcanic productions. The coast-chain has the same character, with immense tracts of volcanic rocks, both ancient and modern, especially obsidian, which is nowhere developed on a greater scale, except in Mexico and the Andes.

In North America, as in the southern part of the continent, volcanic action is entirely confined to the coast and high land along the Pacific. The numerous vents in Mexico and California are often in great activity, and hot springs abound. Though a considerable interval occurs north of them, where the fire is dormant, the country is full of igneous productions, and it again finds vent in Prince of Wales’s Archipelago, which has seven active volcanos. From Mount St. Elias westward through the whole southern coast of the peninsula of Russian America and the Aleutian Islands, which form a semicircle between Cape Aliaska, in America, and the peninsula of Kamtchatka, volcanic vents occur, and in the latter peninsula there are three of great height.

From the similar nature of the coasts, and the identity of the fossil mammalia on each side of Behring’s Strait, it is more than probable that the two continents were united, even since the sea was inhabited by the existing species of shell-fish. Some of the gigantic quadrupeds of the old continent are supposed to have crossed, either over the land or over the ice, to America; and to have wandered southward through the longitudinal valleys of the Rocky Mountains, Mexico, and Central America, and to have spread over the vast plains of both continents, even to their utmost extremity.^[66] An extinct species of horse, the mastodon, a species of elephant, three gigantic edentata, and a hollow-horned ruminating animal roamed over the prairies of North America—certainly since the sea was peopled by its present inhabitants, probably even since the existence of the Indians. The skeletons of these creatures are found in great numbers in the saline marshes on the prairies called the Licks, which are still the resort of the existing races.^[67]

There were, however, various animals peculiar to America, as well as to each part of that continent, at least as far as yet known. South America still retains in many cases the type of its ancient inhabitants, though on a very reduced scale. But on the Patagonian plains, and on the Pampas, skeletons of creatures of gigantic size and anomalous forms have been found, one like an anteater of great magnitude, covered with a prodigious coat of mail similar to that of the armadillo; others like rats or mice, as large as the hippopotamus—all of which had lived on vegetables, and had existed at the same time with those already mentioned. These animals were not destroyed by the agency of man, since creatures not larger than a rat vanished from Brazil within the same period.

The geological outline of the United States, the Canadas, and all the country of the Polar Ocean, though highly interesting in itself, becomes infinitely more so when viewed in connection with that of northern and middle Europe. A remarkable analogy exists in the structure of the land on each side of the North Atlantic basin. Gneiss, mica-schist, and occasional granite, prevail over wide areas in the Alleghanies, on the Atlantic Slope, and still more in the northern latitudes of the American continent; and they range also through the greater part of Scandinavia, Finland, and Lapland. In the latter countries, and in the more northern parts of America, Sir Charles Lyell has observed that the fossiliferous rocks belong either to the most ancient or to the newest formations,^[68] to the Silurian strata, or to such as contain shells of recent species only, no intermediate formation appearing through immense regions. Silurian strata extend over 2000 miles in the middle and high latitudes of North America; they occupy a tract nearly as great between the most westerly headlands of Norway and those that separate the White Sea from the Polar Ocean; and Sir Roderick Murchison has traced them through central and eastern Europe, and the Ural Mountains, even to Siberia. Throughout these vast regions, both in America and Europe, the Silurian strata are followed in ascending order by the Devonian and carboniferous formations, which are developed on a stupendous scale in the United States, chiefly in the Alleghany mountains and on the Atlantic Slope. The Devonian and carboniferous strata together are a mile and a half thick in New York, and three times as much in Pennsylvania, where one single coal-field occupies 63,000 square miles between the northern limits of that state and Alabama. There are many others of great magnitude, both in the States and to the north of them, so that most valuable of all minerals is inexhaustible, which is not the least of the many advantages enjoyed by that flourishing country. The coal formation is also developed in New Brunswick, and traces of it are found on the shores and in the islands of the Polar Ocean, on the east coast of Greenland, and even in Spitzbergen.

Vast carboniferous basins exist in Belgium, above the Silurian strata; and a great portion of Britain is perfectly similar in structure to North America. The Silurian rocks in many instances are the same, and the coal-fields of New England are precisely similar to those in Wales, 3000 miles off.

In all the more northern countries that have been mentioned, so very distant from one another, the general range of the rocks is from north-east to south-west; and in northern Europe, the British isles, and North America, great lakes are formed along the junction of the strata, the whole analogy affording a proof of the wide diffusion of the same geological conditions in the northern regions at a very remote period. At a later time those erratic blocks, which are now scattered over the higher latitudes of both continents, were, most likely, brought from the north by drift ice or currents, while the land was still covered by the deep. Volcanic agency has not been wanting to complete the analogy. The Silurian and overlying strata have been pierced in many places by trappean rocks on both continents, and they appear also in the islands of the North Atlantic and Polar Seas. Even now the volcanic fires are in great activity in the very centre of that basin in Iceland, and in the very distant and less-known island of Jan Meyen.

The average height of the continents above the level of the sea is the mean between the height of all the high lands and all the low. Baron Humboldt, by whom the computation was effected, found that the table-lands, with their slopes, on account of their great extent and mass, have a much greater influence upon the result than mountain-chains. For example, if the range of the Pyrenees were pulverized, and strewed equally over the whole of Europe, it would only raise the soil 6 feet; the Alps, which occupy an area four times as great as that on which the Pyrenees stand, would only raise it 22 feet; whereas the compact plateau of the Spanish peninsula, which has only 1920 feet of mean height, would elevate the soil of Europe 76 feet; so that the table-land of the Spanish peninsula would produce an effect four times as great as the whole system of the Alps.

A great extent of lowland necessarily compensates for the high—at least it diminishes its effect. The mean elevation of France, including the Pyrenees, Juras, Vosges, and all the other French mountains, is 870 feet, while the mean height of the whole European continent, of 1,720,000 square miles, is only 670 feet, because the vast European plain, which is nine times as large as France, has a mean altitude of but 380 feet, although it has a few intumescences, which, however, are not much above 1000 feet high, so that it is 200 feet lower than the mean height of France.^[69]

The great table-land of Eastern Asia, with its colossal mountain-chains, has a much less effect on the mean height of Asia than might have been expected, on account of the depression round the Caspian Sea; and still more from the very low level and the enormous extent of Siberia, which is a third larger than all Europe. The intumescences in these vast plains are insignificant in comparison with their vast area, for Tobolsk is only 115 feet above the level of the sea; and even on the Upper Angora, at a point nearer the Indian than the Arctic Sea, the elevation is only 830 feet, which is not half the height of the city of Munich, and the third part of Asia has a mean height of only 255 feet. The effect of the Great Gobi, that part of the table-land lying between Lake Baikal and the wall of China, is diminished by a vast hollow 2560 feet deep, the dry basin of an ancient sea of considerable extent near ErgÉ, so that this great desert has a mean height of but 4220 feet, and consequently it only raises the centre of gravity of the Asiatic continent 128 feet, though it is twice as large as Germany. The table-land of Tibet, whose mean elevation, according to Baron Humboldt, is 11,600 feet, together with the chains of the Himalaya and Kuenlun, which enclose it, only produces an effect of 358 feet. On the whole the mean level of Asia about the sea is 1150 feet.^[70]

Notwithstanding the height and length of the Andes, their mass has little effect on the continent of South America on account of the extent of the eastern plains, which are exactly one-third larger than Europe. For if these mountains were reduced to powder, and strewn equally over them, it would not raise them above 518 feet; but when the minor mountain systems and the table-land of Brazil are added to the Andes, the mean height of the whole of South America is 1130 feet. North America, whose mountain-chains are far inferior to those in the southern part of the continent, has its mean elevation increased by the table-land of Mexico, so that it has 750 feet of mean height.

The mean elevation of the whole of the New World is 930 feet, and the height of the centre of gravity of all the continental masses above the level of the sea, Africa excepted, is 1010 feet. Thus, it appears that the internal action in ancient times has been most powerful under Asia, somewhat less under South America, considerably less under North America, and least of all under Europe. In the course of ages changes will take place in these results, on account both of the sudden and gradual rise of the land in some parts of the earth, and its depression in others. The continental masses of the north are the lowest portions of our hemisphere, since the mean heights of Europe and North America are 670 and 750 feet.^[71]

So little is known of the bed of the ocean that no inference can be drawn with regard to its heights and hollows, and what relation its mean depth bears to the mean height of the land. From its small influence on the gravitating force, La Place assumed it to be about four miles. As the mean height of the continents is about 1000 feet, and their extent only about a fourth of that of the sea, they might be easily submerged, were it not that, in consequence of the sea being only one-fifth of the mean density of the earth, and the earth itself increasing in density towards its centre, La Place has proved that the stability of the equilibrium of the ocean can never be subverted by any physical cause: a general inundation from the mere instability of the ocean is therefore impossible.