« PreviousContinue »
PHYSICAL STRUCTURE OF THE EARTH.
Structure of the Earth.
Amid the marvels of the world below!
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, bear
ing 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.
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 thickness of the crust of the earth; the points de f g, 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,
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 strata, always succeeding in the same order wherever examined, and teach formation marking a distinct epoch in the history of our planet; each characterized by its own flora and fanna, so that the whole substance which has hitherto been explored, consists of either minerals, i. e. inorganic substances formed by natural operations, or the fossil remains of animals and vegetables, characterizing peculiar and distinct epochs in the history of the globe. “The arrangement of the various formations may be represented by an alphabetical series from a to z, and this order, though it is frequently imperfect, is never inverted. We often miss one, or more, terms in the series, and lose, say the b or h or m, or even several letters in succession, but we never find the b taking the place of the a, or d preceding the c, or any member of the series 'usurping the position of another which ought to go before it; in other terms, we never meet with the entire series of deposits, but those which do occur invariably follow in a regular order of sequence.” We have said that these strata are all either mineral or fossil. Remotely they all are of mineral origin, for all organic substances have been at some time elaborated from inorganic matter by that marvelous principle termed vitality. There are fourteen simple substances which are named below in order, according to their importance, which constitute the chief part of the earth’s surface. The first eight are called simple Non-Metallic substances.
1. Oxygen, 5. Sulphur,
These eight simple substances by their union with certain metals, which are hence called metallic bases, and also by union with each other, form by far the greatest amount of all the matter, organic, or inorganic, solid, or liquid, or gasseous, which is known to exist on the surface of the earth. The metallic bases alluded to are.
1. Silicium, 4. Sodium,
We cannot here describe each of these elementary bodies, this
will be found in most treatises upon chemistry; suffice it that a union of silicium and oxygen forms nearly one half the solid part of the earth's crust, being a chief ingredient in all the principal rocks. It appears nearly pure in the state of transparent rock crystal, and is the chief ingredient of our ordinary flints and sands. Aluminium in combination with oxygen, is the base of the various clays and clayey slates. Potassium in combination with oxygen, and also sodium with oxygen, constitute very important ingredients of the rocks and earths under the well known forms of potash and soda, and the latter in combination with chlorine forms muriate of soda or common salt, so widely prevalent in the ocean, and in beds of rock salt. Magnesium is the base of manganese a chief ingredient of the chalks, and magnesian limestones; and calcium is the metallic base of lime, and is found in abundance in the various limestones and gypsums. Bosides the simple substances named, we have the metallic minerals, which are usually found in beds or veins. The most of these are so well known that they are recognised at a glance. Iron is the most useful and most abundant, when combined with sulphur it crystalises in cubes, is of a bright yellow, and is often mistaken for gold; this variety is termed iron pyrites. Iron is likewise found in combination with oxygen and carbon, and occasionally nearly pure. Lead is a well known metal occurring principally in union with sulphur, under the form called galena, in cubic crystals. Copper is found in combination with oxygen and sulphur, and also native, or pure; in combination with carbon and oxygen it assumes beautiful tints of blue and green. Tin, zinc, and manganese are too well known to need any particular description here. It is said that tin is only found in the primitive or lowest order of rocks, and that the tin mines of Cornwall extend many hundred feet under the sea, and that the noise of the waves and the rolling of the pebbles can be distinctly heard. We need not describe the precious metals, silver, gold, and platina, as everybody is familiar with their general properties. The various substances which form the crust of the earth, and which have been investigated by the persevering energy of man, are arranged into two great classes, which embrace all the various soils, sands,