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Perhaps the best which we can adopt, as embracing all the others, without their minute subdivisions, is the following:

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5. BASALTIC, AND VOLCANIC ROCKS.
6. DILUVIAL, AND ALLUVIAL DEPOSITES.

PRIMARY ROCKS.

These compose the great frame, or ground-work of the globe. They form the most lofty mountains, and at the same time extend downward below all other formations. One of the principal rocks of this class is granite. This is a compound rock, being composed of three distinct minerals aggregated into a solid form. These are quartz, felspar, and mica. Quartz has commonly a white color, a glassy lustre, and does not divide into layers when broken. It often forms a large proportion of the granite. Felspar has a yellowish, or milk white color, and when broken, often divides into layers of considerable thickness, with smooth shining faces. Mica is also sometimes white, but more commonly of a dark green color. It consists of thin flexible leaves, adhering slightly together, and easily separable by the nail. This is well known under the name of isinglass, and when in large plates is used for economical purposes, as the dead lights for ships, windows for stoves and lanthorns, &c. Granite never consists of strata, or layers, like gneiss and mica-slate. These minerals differ greatly in their respective proportions in different rocks. They also differ widely with respect to size, some granites being composed of crystals, or grains, a foot in diameter; while in others the grains are no larger than those of sand.

The other Primitive rocks, are Gneiss, Mica-slate, Clay-slate, Primitive Limestone, Porphyry, and Sienite; to which some add several others.

This whole class is generally crystalline in its struc

ture, and never contain the fragments of other rocks, or any organized substance.

Gneiss, and mica-slate are composed of the same materials as granite, but differently arranged. They are also generally composed of much smaller grains than granite. In Gneiss the felspar and quartz are aggregated closely together, forming strata, or layers, between which intervene scales of mica. Hence gneiss is a stratified rock, and when broken at right angles with the strata, presents a striped appearance, the quartz and felspar being nearly white, while the mica is deep green or black.

Mica-slate is chiefly composed of quartz and mica, the felspar being in only small quantities, or in some instances nearly absent. The quartz is commonly in fine grains, and the mica usually predominates, or at least is much the most apparent. Some specimens of this rock appear to be almost entirely composed of small scales of mica, closely adhering together.

Mica-slate differs from gneiss in containing a less proportion of felspar, and in being more distinctly stratified, or slaty in its structure. It is readily divided into layers, or tables, by means of wedges, and is extensively employed for economical purposes, especially for flagging the side walks of cities.

Gneiss is intermediate between granite and mica-slate in its structure, and is often found interposed between these rocks, lying over the former, and under the latter. Indeed these rocks pass by insensible degrees into each other, the granite gradually becoming stratified runs into gneiss, while the gneiss becoming fissile forms mica-slate. These three are called granitic rocks, and form together a great proportion of the solid crust of our globe.

The adjoining wood cut from Daubuisson, represents the most common relative positions of granite, gneiss and mica-slate, as they occur on the earth.

The centre or middle mass, 1, projecting high above. the side strata, is granite. The flanking planes, 22, are gneiss, appearing as though they had been elevated to their present situation by the tremendous force which lifted up the granite. The mica-slate, 3, 3, is seen resting against the gneiss. The two latter rocks have the appearance of once having been in a horizontal position,

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the mica-slate being superincumbent on the gneiss, and this on the granite; and we shall see in another place that this was undoubtedly the case. g is a great bed of quartz, included in the micaceous beds, and being much less subject to the disintegration by the weather, rises above the mica. 44, are beds of clay-slate, or roof-slate, on the outside of the mica-slate. 5, is an overlaying mass of porphyry, resting on the mica, and clay-slate. 6, a small bed of mica-slate resting between the central peaks of granite, with the strata bent and sloping in opposite directions, forming a dish-like cavity. Above 7 is seen a bed of clay and gravel in strata, lying nearly horizontal on the upright edges of the clay-slate, demonstrating their subsequent and independent formation.

In many instances there is sufficient proof exhibited by the rocks themselves, that the primitive strata were once in a horizontal position, and that they owe their present vertical position to a force exerted from below, and by which the granite, being elevated, has raised up the once superincumbent rocks, and given them their various inclinations. This subject has already been examined, under" Elevations of Continents from the Sea."

Clay-slate. Roof-slate. This rock is exceedingly fissile, and being divided into thin plates, is in very general use for the roofing of houses; its appearance, therefore, is too generally known to need description.

This is the most distinctly stratified of all the primitive rocks, and it is a singular circumstance, that its strata are commonly very highly inclined,-sometimes nearly, or quite vertical. This rock is associated with granitic rocks, being often superincumbent on mica-slate.

Primitive Limestone. This is called primitive, to distinguish it from the secondary, or that which has been more recently formed; for limestone is of all ages, from that which is now forming at the mouth of the Rhone, to that which has the antiquity of the granitic mountains.

Primitive limestone is crystalline in its structure, and is found associated with granite, gneiss and mica-slate, being often intermixed with the latter, or alternating in layers with it. No organic remains are found in this rock, and hence, like granite, it is supposed to have been formed before the creation of living beings. When white and pure, it is known in the arts, under the name of statuary marble, of which the finest specimens of ancient as well as modern sculpture are made. It is found particularly in Italy, Switzerland, and the Grecian Archipelago. The Carara marble is a primitive limestone.

Secondary Limestone contains shells and other organic bodies-is compact, and not crystalline in its structure, and is associated with secondary rocks. Thus may the two kinds be distinguished.

Porphyry derives its name from a Greek word, signifying purple, because the first rock to which this name was applied had a purple color. At present, however, any rock having a compact, or paste-like base, with imbedded crystals, is called by this name, whatever its color may be.

Porphyry has the appearance of having once been in the form of a soft paste, into which crystals of various kinds, but most commonly felspar, have by some unknown means been introduced. When associated with granite, porphyry is considered a primitive rock, but is sometimes secondary, and sometimes volcanic. It may, perhaps, be considered as the connecting link between granitic rocks, and those of igneous origin.

The columns of some of the most ancient and splendid edifices were made of porphyry, of which the remains are still in existence. The great hardness of this rock; the high polish which it is capable of bearing, and the variety and beauty of the colors which it often presents, afford a combination of qualities for splendid and enduring architectural purposes which is found in no other mineral body. But the labor of forming pillars thirty or forty feet in height, and five or six feet in diameter, of this ma

terial, such as the ancients constructed, is much too great and expensive for the present age.

Porphyry, though not an uncommon rock, seldom occurs in extensive formations like granite and limestone.

Sienite. This rock is composed of quartz, felspar and hornblende. It may be considered as a granite in which the mica is replaced by hornblende; it, however, sometimes contains small portions of mica. Its structure is granular like that of granite, and its prevailing color is yellowish white, mottled with black, giving it a grey appearance. The city of Boston contains many magnificent columns of sienite. It is associated with granite, into which it gradually passes, as the mica takes the place of the hornblende.

IGNEOUS ORIGIN OF GRANITE.

It was formerly believed that granite was of aqueous origin, that is, that the materials of which it is composed were first dissolved in water as preparatory to their assuming that solid and crystalline form, which we see at the present time. Now chemistry has long since taught us that no substance in the labratory of art, nor so far as is known, in that of nature, ever assumes the crystalline form until it has been dissolved in some kind of fluid; and indeed a single consideration would seem to show, beyond all question, the necessity of such solution, for otherwise there could be no motion among the particles of which the crystal is formed, and without motion it is equally certain that these particles never could take their places according to the laws of affinity, or in other terms, never could assume crystalline forms.

The kind of fluid in which the particles are dissolved, it is obvious, must depend on the kind of substance. Thus some substances are soluble in water, others in acids, and others in caloric. Now, although the materials composing granite are scarcely soluble by any artificial means, still there is no doubt but under a very high temperature, with the combined aid of pressure, they would be soluble in water, or in caloric alone, and the phenomena, as we shall see, afford conclusive evidence that the latter was the solvent,

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