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sive flat tracts which are almost entirely destitute of small mountain groups. III. To the bottom of the sea, belong the flat, the rocky bottom, shoals, reefs, and islands. Rocks are those stony masses which form a portion of the substance of the globe, and are generally disposed in ranges, like mountains, but in some few instances are found to exist in large and separate masses.

Rocks rich in gems, and mountains big with mines,
That on the high equator ridgy rise,

Whence many a bursting stream auriferous plays.

THOMSON.

Rocks are divided into five classes, which are called formations; as, primitive, transition, fletz, alluvial, and volcanic. (1.) Primitive, are the class of rocks on which all others rest, whose texture is more or less crystalline, a quality denoting previous chemical solution. They comprehend granite, gneiss, micaslate, talc-slate, hornblende slate, syenite, porphyry, serpentine, and lime-stone. (2.) Transition rocks are principally composed of chemical productions. Lime-stone occurs more frequently in this than in the preceding class. These rocks were formed during the transition of the earth into a habitable state; they differ from the primitive in the variety of their colour, and by containing the remains of marine animals. (3.) Fletz rocks disposed in flat or horizontal strata, after the creation of animals and vegetables, the remains of which are often found in the substance of these rocks. (4.) Alluvial rocks are formed by the component parts of previously existing rock, separated

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by the influence of air, water, and change of temperature, and deposited in beds. Sand, gravel, loam, and petrifactions of animals and vegetables, are often found in this class. (5.) Volcanic formations are pseudo-volcanic, or such minerals as are altered in consequence of the burning of beds of coal in the neighbourhood; and true-volcanic, or such as are actually thrown from the crater of the volcano. The volcanic productions are: (1.) Pumice-stone, a kind of glass, in the form of small greyish, white, and exceedingly brilliant filaments. It is often lighter than water. (2.) Lava, the burning matter which runs down, in such prodigious quantities, from volcanos, when in a state of eruption,, and often extends to a great distance. This matter is a semi-vitrified substance, and appears of a blackish cast. (3.) Basaltes is blackish and opaque, and may, by the action of heat, be converted into glass, of a very beautiful black colour. Of some kinds, such as that known under the name of touchstone, the grain is exceedingly fine.

Decomposition of Rocks.-The expansion of water in the pores or fissures of rocks by heat, or congelation, is a physical cause of the separation of their parts. The solvent power of moisture exerted upon alkaline or calcareous matter, in rocks, is another cause of their decomposition. Electricity, which is shown, by experiments with the voltaic apparatus, to be a most powerful agent of decomposition, seems to assist in all these changes; electrical powers being almost constantly exhibited in the atmosphere. The production of a bed for vegetation is effected by

the decomposition of rocks. As soon as the rock begins to be softened, the seeds of lichens, which are constantly floating in the air, make it their resting place. Their generations occupy it, till a finely divided earth is formed, which becomes capable of supporting mosses and heath: acted upon by light and heat, these plants imbibe the dew, and convert constituent parts of the air into nourishment. Their death and decay afford food for a more perfect species of vegetable; and, at length, a mould is formed, in which even the trees of the forest can fix their roots, and which is capable of rewarding the labours of the cultivator.

The decomposition of rocks tends to the renovation of soils, as well as their cultivation. Finely divided matter is carried by rivers from the higher districts to the low countries, and alluvial lands are usually extremely fertile. The quantity of habitable surface is constantly increased by these operations; precipitous cliffs are gradually made gentle slopes, lakes are filled up, and islands are formed at the mouths of great rivers. In these series of changes, connected with the beauty and fertility of the surface of the globe, small quantities of solid matter are carried into the sea; but this seems fully compensated for by the effects of vegetation in absorbing matter from the atmosphere, by the production of coral rocks and islands in the ocean, and by the operation of volcanic fires.

What does not fade? the tower, that long had stood
The crash of thunder and the warring winds,
Shook by the slow but sure destroyer, time,
Now hangs in doubtful ruins o'er its base;

And flinty pyramids and walls of brass
Descend; the Babylonian spires are sunk ;
Achaia, Rome, and Egypt, moulder down.
Time shakes the stable tyranny of thrones,
And tottering empires rush by their own weight.
This huge rotundity we tread grows old;
The sun himself shall die, and antient night
Again involve the desolate abyss.

ARMSTRONG.

The science of geology, independently of the healthy employment it affords, is of great importance in a practical point of view. It very nearly concerns the miner, engineer, and drainer, and even the farmer and architect; and discloses a variety of indications highly useful in their respective pursuits. To the miner, the rocks containing metallic veins and coals; to the engineer, the association of hard rocks with soft; to the drainer, the intersection of a country by hard dykes, or veins impermeable to water; to the farmer, the best places for finding lime-stone, marle, and clay; and to the architect, the most durable stones for buildings. The person who is attached to geological inquiries can scarcely ever want objects of employment and of interest. The ground on which he treads-the country which surrounds him-and even the rocks and stones removed from their natural position by art, are all capable of affording some degree of amusement. Every new mine or quarry that is opened, every new surface of the earth that is laid bare, and every new country that is discovered, offers to him novel sources of information. In travelling, he is interested in a pursuit which must constantly preserve the mind awake to the scenes presented to it; and the beauty, the majesty,

and the sublimity of the great forms of nature, must necessarily be enhanced by the contemplation of their order, their mutual dependence, and their connexion as a whole.

Oryctology is the science which teaches the natural history of those animal and vegetable substances which are dug out of the earth in a mineralized state. By this science, we obtain not only a knowledge of the peculiar beings which dwelt on this planet in its antediluvian state, but we also acquire a more correct knowledge of the structure of the globe itself. Among these we find the remains of several animals not known to exist: such as the BELEMNITE, part of a chambered shell, but formerly thought to be a thunderbolt; the ENCRINITE, a petrified animal, formerly termed a stone lily; the CORNU AMMONIS, a shell, formerly considered as a petrified snake; the MAMMOTH, an animal resembling the elephant, but possessing grinders much like to those of carnivorous animals, with numerous others, yielding additional proofs of the wisdom and power of the great Creator of all things.

But we must not confine ourselves while on this subject to the notice of the curious forms and wonderful nature of these bodies; but must consider them with respect to the intelligence which they may convey respecting the formation of this globe. It was first remarked by Mr. WILLIAM SMITH, that certain fossils were peculiar to certain strata, and by extending his observations very widely, bearing this principle in view, he was able to determine that the identi

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