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

ta, always succeeding in the same order wherever examined, and each 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 bor 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.

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,
2. Aluminium,

3. Potassium,

4. Sodium,

5. Magnesium,
6. Calcium.

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. Besides 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,

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gravels, clays, limestones, coals, slates, and granites; these two classes, are the stratified, and unstratified rocks. In the former, are included those portions of the crust of the earth which exhibit a sedimentary character, i. e., evidence of deposition through the agency of water. It is supposed that all rocks were once deposited in this manner, but that the present crystaline form of some of them is owing to heat, hence the unstratified, are sometimes termed the igneous or the plutonic rocks; and the strata which happen to intervene between them, being partly changed in their character, yet not wholly so, are termed the metamorphic, or transformed rocks; and those rocks resembling lavas scoriæ, and other substances, emitted by burning mountains, still in activity, are called volcanic. When we assume that all the igneous or plutonic rocks, such as granite, sienite, and the like, are of sedimentary origin, we speak hypothetically, they are supposed to be so; for rocks, which are well known to present evidence of aqueous origin, become crystaline, and lose the marks of stratification under pressure, and by the influence of heat; thus, chalk has been converted into marble. Sir James Hall, exposed pounded chalk to intense heat, under great pressure, and it was fused, not into lime, but into crystaline marble; even the shells inclosed in the chalk underwent the same transmutation, yet preserved their forms; and where ancient streams of lava have traversed chalk, the latter invariably possesses a crystaline structure, and a series of changes from a loose earthy deposit, to compact volcanic lava, may be traced in numerous instances, so as to leave no doubt of the former aqueous origin, and the sedimentary deposit.

The crystaline rocks, such as granite, sienite, porphyry, serpentine, and greenstone, are generally termed the ancient or earliest rocks, as they are uniformly found underlying all the other strata; they were hence, named hypogene, under-lying, by Mr. Lyell. It is now however, ascertained that they belong to no particular age or epoch, exclusively; for granite is found occurring at comparatively modern, as well as ancient epochs, overlaying the other strata precisely in the same manner as masses of volcanic rock recently ejected, spread out upon the soil below. The difference in character between the modern lavas, and

the lower igneous rocks, is doubtless to be attributed to their formation upon the surface, instead of under pressure. The term hypogene includes the plutonic, and metamorphic rocks.

The sedimentary rocks are termed fossiliferous, or fossil-bearing, in distinction to the igneous, or plutonic rocks, in which, by the action of heat, all fossil remains are wanting. The stratified rocks were originally deposited in a horizontal position as represented in this engraving, but they are rarely seen perfectly hori

zontal, hence, it is inferred, that subsequently to their deposition, they have been subjected to a variety of disturbing causes, by which they have been made to assume all inclinations to the horizon. Occasionally they are uplifted to an almost vertical position, as in the following illustration, exhibiting the strata on which Powis Castle is built; when by being thus upheaved, the

edges of the strata are denuded, or uncovered, they are said to crop out, when this occurs, it will be found that the strata succeed each other in regular order, as we have already mentioned. When strata crop out, it is apparent, that having been elevated by