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. 4 4, 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 of thirty or forty feet in height, and five or six feet in diameter, of this ma

tenal, 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 gray ap pearance. 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 laboratory 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 sol

vent, and that the materials composing granite were once in a melted state.

The igneous origin of granite is satisfactorily proved, from the phenomena of its veins:-from the calorific effects of these veins on the walls of the rocks, through which they have protruded;-from the intrusion of granitic matter between the strata of various rocks through which such veins have been forced, and lastly, from the passage of known igneous rocks into granite.

The igneous origin of trap rocks has long been acknowledged by all competent geologists, but the general agreement that granite had the same origin is only of recent date. The proofs however of the origin of both are nearly the same.

Under the "Origin and phenomena of Trap Rocks," it will be seen that dykes or veins of basalt often protrude through the strata of other rocks, and that where they come into contact with these strata, the effects of heat are

always apparent. The illustrations by diagrams, also prove that these veins, or dykes, were forced through the fissures, or spread between the strata of the rocks, while the former was in a soft or semifluid state. The same phenomena are found to attend veins of granite which traverse other rocks, there being every indication that these veins were forced up from below in an ignited and softened state.

Fig. 16.

The diagram Fig. 16, will show the manner in which granite sometimes traverses stratified rocks. This drawing is from Dr. Macculloch's representation of granite veins passing through gneiss at cape Wrath in Scotland. These veins, it may be observed, intersect each other in various directions,

and are curiously branched and contorted. The mass of gran. ite below the stratified gneiss, is also apparent, and as the veins end before reaching the surface of the gneiss, we cannot

but infer that they were forced up in a softened state from the underlaying granite with which their trunks are incor porated. Similar instances, that is, of granite veins travers ing stratified rocks, and also rocks of granite, are known to occur frequently and in various parts of the world. In Europe such cases were formerly considered singular and important phenomena, and as they went to prove the igneous origin of granite, they were described with great prolixity and exactness. But the progress of observation has shown that granitic veins are quite common, and that particularly in mica-slate, examples may be seen in almost any place, where circumstances allow the rock to be examined a few yards below the surface, and often on the surface itself.

In this country, Prof. Hitchcock of Amherst College, in his Report of the Geology of Massachusetts, has described and figured many such cases; some of which we shall take the liberty of inserting at this place.

Fig. 17.

Fig. 17, (fig. 11, in Prof. Hitchcock's work,) represents a vein of granite protruding through strata of hornblende slate. It occurs at Ackworth, New Hampshire, and is a remarkable locality of beryls, rose quartz, and crystallized

mica.