dium, molybdenum, tungsten, columbium, antimony, uranium, cerium, bismuth, titanium, tellurium, copper, lead, mercury, silver, gold, platina, palladium, rhodium, osmium, and iridium. These metallic elements are again divided into three orders, the first twelve being the bases of the alkalies and earths; the next twentyone being metals whose oxides are not reduced by heat alone, and the remaining eight, whose oxides are reduced by a red heat.

From these fifty-four elementary substances are formed all the beautiful varieties of terrestrial objects: nor is there any thing very wonderful or mysterious in this fact; since, as we have seen, any given two of them, if made to unite in different proportions, are capable of producing the most opposite substances. Thus nitrogen and hydrogen, combined in certain proportions, form the vital air which we breathe; the same elements, combined in another proportion, produce an intoxicating gas; and again, in still another, produce aquafortis, which is a deadly poison. It is also to be observed that new substances, thus produced, united with each other, give rise to new compounds, which are susceptible of being combined, and so on through an almost infinite diversity of chemical union. From recent experiments in chemistry, however, it has been suggested that all substances whatever are but modifications of one primitive element; but the absolute truth of this startling theory remains to be practically demonstrated.

XIII.-23

THIS science proposes to investigate the natural history of the earth-especially the general structure of what may be called its crust, or shell. It does not entirely overlook the internal strata, or even the nucleus, of our globe; but as these are beyond our inspection, it is possible to offer little more than speculations respecting them.

This noble science is of modern date. Certain theories had, indeed, been offered upon this subject;

but the extravagance of these, proceeding, as they often did, from men of the highest talent, affords humiliating lessons as to the absurdities in which the master-spirits of our race may be involved, when their footsteps do not follow the paths of experiment and observation. The great mathematician Kepler attempted to prove that the earth was a vast animal; the tides he regarded as occasioned by the heavings of its prodigious lungs. ato and the Stoics seem to have entertained a similar opinion. Whiston, the English divine, considered the earth as produced by the condensation of a comet, and the deluge as occasioned by the visit of one of those erratic orbs.

Other theorists have ascribed the origin of the globe to fragments which have fallen successively from the heavens, in the form of aërolites. Our own Captain Symmes, of Cincinnati, seriously maintained that the earth was hollow, and inhabited, and that the interior was accessible by openings at the poles. He brought a vast deal of learning to the support of his theory, and even undertook to equip an expedition for the purpose of exploring the polar regions, in order to determine the question by observation. Perhaps, however, the palm of absurdity must be awarded to Voltaire, who accounted for the immense masses of sea-shells, found upon the mountains of Geneva, by supposing them to have been thrown there from the wallets of pilgrims in the holy wars! Such are some of the follies into which the highest intellect may be led, either by a partial observation of facts, or the adoption of a false philosophy.

The application of Lord Bacon's rule, which instructs

us to collect facts first, and form theories afterwards, has exploded the vain speculations of former geologists and resulted in the establishment of the modern science upon a permanent and secure basis. The first person who pointed out the proper mode of investigation, in the pursuit of geological knowledge, was William Smith, a land surveyor, of Bath, England, who, in constructing roads and canals, observed that the same strata gave the same fossils, and that strata and fossils were always identical. This was a key; and no study ever became more popular, and raised itself into universal estimation more suddenly. Parkinson, Cuvier, Mantell, Brogniart, Sedgwick, Buckland, Murchison, Greenough, Lyell, Philips, Silliman, and the learned societies throughout Christendom, have been active in exploring this interesting field of knowledge.

NATURE OF THE CRUST OF THE EARTH.

The greatest thickness of the superficial crust of the globe- that is, of the mass of solid materials which the ingenuity of man has been able to examine, from the highest mountain-peaks to the greatest natural or artificial depths is estimated at about 10 miles. As the earth is nearly 8000 miles in diameter, the entire series of strata hitherto explored is, therefore, but very insig nificant, compared with the magnitude of the globe; bearing about the same relative proportion as the thickness of paper to an artificial sphere a foot in diameter; the inequalities and crevices in the varnish of such an instrument would be equal, in proportionate size, to the highest mountains and deepest valleys.

As a thickness of 100 miles so far exceeds that of

the whole of the strata that are accessible to human observation, we cannot doubt that disturbances of the earth's surface, even to ten times the depth of those which come within the scope of geological inquiry, may take place, without in any degree affecting the entire mass of the globe. If these facts be duly considered, the mind will be prepared to receive one of the most startling propositions in modern geology— namely, that the highest mountains have once been the bed of the sea, and have been raised to their present situations by subterranean agency,- some slowly, others suddenly; but all, geologically speaking, at a comparatively recent period.

The superficial crust of the globe is composed of numerous layers and masses of earthy substances, of which, combinations of iron, lime, and silex, or flint, constitute a large proportion; the latter forming 45 per cent. of the whole. Those strata which have been deposited the latest, bear evident marks of mechanical origin, and are the water-worn ruins of older rocks; as we descend, materials of a denser character appear, which also exhibit proofs of having been subject to the action of water; but when we arrive at the lowermost in the scale, a crystalline structure generally prevails; and while, in the newer strata, trees, plants, shells, and other remains of animals and vegetables, are found in profusion, in the most ancient rocks all traces of organic forms are absent.

CLASSIFICATION OF ROCKS.

In casting a casual glance over the broken and diversified surface of the globe, its materials might present