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PART III,

PHYSICAL STRUCTURE OF THE EARTH,

C H A P T E R 1.
Structure of the Earth.

“Ye mighty ones who sway the souls that go
Amid the marvels of the world below!
Ye, silent shades, who sit and hear around!
Chaos! and streams that burn beneath the ground!
All, all forgive, if by your converse stirred,
My lips shall utter what my ears have heard;
If I shall speak of things of doubtful birth,
Deep sunk in darkness, as deep sunk in earth.”

z- Virgil.

WE have before shown that our globe is a planetary orb of a few thousand miles in diameter, and of a spheroidal shape, the difference between the polar, and equatorial diameters being twenty-six miles. The mean density of the earth, is about five times that of water, the interior being double that of the solid superficial crust, hence if the interior of the earth be cavernous, - its crust must be composed of very dense materials. The crust, or outer covering of the earth, significantly called “Erdrinde,” or Earth-rind, by the Germans, is that part to which our investigations are naturally directed. The greatest thickness of this superficial crust, which man has been able to explore, estimated from the highest mountain peaks, to the greatest natural or artificial depths, does not exceed ten miles; this, in comparison with the diameter, 8000 miles, is a distance, utterly insignificant, bearing about the same relative proportion, as the thickness of this paper to an artificial sphere a foot in diameter. The inequalities and crevices in the varnish of such a sphere, would proportionately represent the highest mountains, and deepest valleys. In the following diagram, from the Penny Cyclopedia, the relative proportions of the crust of the earth, and the inequalities of its surface, as compared with the mass of our planet, are attempted to be shown.

i i The line from e to k, represents a depth of 500 miles, to the point i, a depth of 100 miles, and to the line b, 45 miles above the surface, the supposed limit of the earth's atmosphere. The dark line represents a thickness of ten miles, the estimated thickmess of the crust of the earth; the points d e fg, indicate the altitudes of the highest mountains in the world. The highest peak in Europe, being Mont Blanc, which is 15,660 feet above the level of the sea; and in America, Mount Sorata, Andes, 25,400 feet, and in Asia, Chumularee, Himalayah, estimated at 29,000 feet, being more than five miles of perpendicular altitude. The depth of the sea is shown by the line a h, at the extremity of the arc. When we consider that the altitude of the highest mountains bears so small a proportion to the probable thickness of the earth’s crust, we will be prepared to admit the possibility that they might once have been the bed of the ocean, and may have been raised to their present situations by subterranean agency,

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EXTENT OF SURFACE. 179

The external crust, or covering of the earth, is composed of a vast amount of substances which we shall more fully describe hereafter, but which, under the indefinite but convenient terms of rocks and earth, embracing every variety of element, and combination, are familiar to every one. Although of such microscopic value as regards the dimensions of the globe itself, yet the crust upon which we are located, is of infinite importance to man. With its alternations of land and water, of valleys and mountains, it is the seat of vast empires, and the storehouse of the wealth of nations. The surface of the earth has been computed to contain one hundred and fifty millions of square miles. about three-fourths of which are covered by seas, and another large proportion by bodies of fresh water, by polar ice, and eternal snows; so that, taking into the estimate the sterile tracts, the forests, the barren mountains, the bogs, morasses, &c., scarcely more than one-fifth of the globe is fit for the habitation of man. The area of the Pacific ocean alone, is estimated as equal to the whole surface of the dry land, hence, if the waters of the globe were uniformly distributed over its surface, the inequalities being leveled, the whole earth would be covered with water to a depth of about three feet. The present arrangement of continents and islands cannot therefore be supposed to have always existed, indeed, there is abundant evidence to show that all those parts, which we call dry land, have at some very remote period been under water, and that the soil upon which we now tread, is composed of regular strata, deposited by water. It is but a short period since the utmost ignorance prevailed as to the structure of the planet which we inhabit. It was accustomed to be looked upon as a mass of confusion, the chaos of old, where, in incongruous masses, were heaped the various substances of which it was composed, and where antagonistic forces were striving confusedly together.

It was true that rocks were found at some places upon the surface, and not at others, but this was regarded as mere matter of chance, no one supposed any order, or any definite arrangement. It was reserved for modern science to show that the crust of the earth from its surface downwards, is composed of regular stra

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