conical half-naked peak, chiefly of white granite, shooting up about 300 feet above the surrounding country. This is the hill represented below, (Fig. 17,) as seen on its northwestern side, along which the road passes. The prevailing rock in the vicinity is gneiss; but in this elevation it is chiefly hornblende slate, traversed by an enormous granite vein, a, and exhibiting at least two protruding masses, b, and c, of granite. The vein varies from one half, to four rods in thickness, and the mass b, is four or five rods across c, is only ten feet wide. The general direction of the laminæ of the slate is north and south, and the dip from 15° to 20° east: but we have here the most decisive marks of its having been irregularly upheaved, and disturbed by the protruding granite. Near the foot of the hill, the slate is bent upwards, so that the chord of the curve is several rods long. But it is a curious fact, that the axis of the elevating force seems not to have coincided with the direction in which the vein was erupted. For the highest point of the curve of elevation, near the foot of the hill, is to the right of the vein at h; and as we ascend the hill we find the slate curved upwards near the vein more and more, as is shown by the drawing. Indeed, the granite of the vein seems to lie on the elevated edges of the slate; so that the lower side of the vein dips northeasterly; and does not cut the slate perpendicularly. These facts would seem to evince, that the vein made its way through the slate, not along the line of the greatest pressure, but on the north side of it; probably because there the slate yielded most readily. We may suppose the melted granite below to have gradually elevated the slate, until at length it burst its way laterally through the rock. Such cases, I believe, do sometimes occur in existing volcanoes. The masses of granite b, and c, are probably other examples in which the molten matter bursts its way laterally through the slate. And it is an interesting fact in regard to the mass b, that in some places it still projects over the slate several feet, forming in fact an overlaying mass. Instances of this kind I have rarely met with in the granite of New England." Page 480.

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Fig. 18, also from Prof. Hitchcock's work, represents a nearly perpendicular ledge of mica-slate in Conway, Mass. The strata as shown by the drawing, are much contorted. indicating disturbance during their deposition, or while they were in a soft and yielding state. a, a, are strata of com

mon mica-slate: b, is a stratum of amphibolic slate. The whole surface exhibited is fifteen feet long and eight feet high. Through this ledge runs a vein of fine grained granite a foot wide.

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The object of giving this sketch," says Prof. Hitchcock, "is to show that this vein has produced no derange. ment of the mica-slate: for the different particles of that rock occupy the same relative position on the different sides of the vein. Hence the vein was introduced subsequently to the consolidation of the slate; and probably it was injected into an open fissure."

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Dr. Hibbert describes the manner in which granite has gradually passed into basalt in one of the Shetland islands. The basalt extends from the island of Mickle Voe northwards to Roeness Voe, a distance of twelve miles. On the west of this there is a considerable mass of granite, and the transition from the one into the other is thus described. Not far from the junction we may find, dispersed through the basalt, many minute particles of quartz. This is the first indication of an approaching change in the nature of the rock. In again tracing it still nearer the granite, we find the particles of quartz dispersed through the basalt, becoming still more numerous and larger, an increase of magnitude even extending to every other description of particles. The rock may now be observed to consist of separate ingredients, of quartz, of hornblende,

felspar, and greenstone; the latter substance, (greenstone,) being a homogeneous commixture of hornblende and felspar. Again, as we approach still nearer the granite, the disseminated portions of greenstone disappear, their place being supplied by an additional quantity of felspar and quartz. The rock now consists of three ingredients, felspar, quartz, and hornblende. The last change which takes place, results from the still increasing accumulation of quartz and felspar, and from the proportionate dissemination of hornblende. The hornblende eventually disappears, and we have a well characterized granite, consisting of two ingredients, felspar and quartz." Ed. Journal of Science, vol. i. p. 107.

We see, from these examples, that granite has been forced from below into the fissures of other rocks which were superincumbent, consequently, which were deposited after the granite was formed. In several instances it may be observed also, that the granite does not reach the surface, by which it is proved that these veins could not have entered from above, a theory long maintained by those who claimed that granite was of aqueous origin. Besides, the indications of fusion which these veins present, the passage of granite into basalt, a rock which all agree bears the marks of fire, is additional evidence that they had a coinmon origin.

But if we consider granite veins to have forced their way from below, in a state of igneous fusion, then we might expect, that when the mass came into contact with stratified rocks, the strata would be separated, and that the fluid matter would run between them, at least to a short distance, and especially near the surface, where the pressure would present little resistance to the separation of the strata. Now this is precisely what is known to have happened in numerous instances, one of the most striking examples of which occurs at Glen Tilt, in the Grampian mountains in Scotland.

At this place, veins of red granite are seen branching out on the northern side of the glen, from the principal mass, and meeting the slate and limestone which forms the southern side. The granite veins run in all directions, intermingling with, and disturbing the strata of the other rocks, in such a manner as to prove, not only that the granite was in a fluid state at the time of its intrusion, but also, that it was forced up with great violence

Fig. 19.

a

The diagram, Fig. 19, from Dr. Macculloch, represents the appearance of these rocks. "The granite at this locality," says Mr. Lyell, "often sends forth so many veins as to reticulate the limestone and schist, the veins diminishing towards their termination to the thickness of a leaf of paper, or a thread. In some places fragments of granite appear entangled, as it were, in the limestone, and are not visibly connected with any larger mass; while sometimes, on the other hand, a lump of the limestone is found in the midst of the granite;" a, granite, b, limestone, c, argillaceous schist.

The ordinary color of the limestone at Glen Tilt is lead blue, and its texture large grained; but where it approximates to the granite, particularly where it is penetrated by the smaller veins, the crystalline texture disappears, and it assumes an appearance exactly resembling hornstone. This change was undoubtedly produced by the heat of the intruding granite.

These facts and circumstances are considered sufficient to show the igneous origin of granite, though an abundance of others of a similar nature might be adduced from authors.

GRANITE OF DIFFERENT AGES.

All the older geological writers believed that granite was the primitive rock of our globe, and the one on which

all others reposed. They also considered this rock as everywhere of a similar age, the idea of successive forinations of granite having never until recently been advanced. These opinions were founded on the general facts, that this rock lies beneath all others, and that it contains no organic remains, which facts even at the present day we must acknowledge to be generally true. More extensive observations have, however, shown many exceptions to these facts, there having been discovered instances where granite not only penetrates through, and reposes on stratified rocks, but also where the rocks invaded by it contain organic remains. Thus Dr. Macculloch describes a considerable mass of granite in the Isle of Sky, which is incumbent on limestone, and shale. The limestone at some distance from the granite contains shells, but in its immediate vicinity, no shells appear, the limestone being converted into pure crystalline marble. This change, as well as the destruction of the shells, is attributed to the heat of the granite at the time of its protrusion.

In different part of the Alps, similar phenomena occur, where, according to the observations of Beaumont, and others, granite is seen penetrating through secondary strata, which contain belemnites, and other fossil organic remains.

In Norway, also, Von Bush discovered a mass of granite overlaying a bed of secondary limestone, containing a variety of fossil shells.

These and other instances of the kind, must however be considered as exceptions to a general rule, there being no doubt, but the granite which universally forms the deeper portions of the crust of our globe, is the eldest of our rocks.

DIFFERENCE BETWEEN IGNEOUS ROCKS.

After having shown that granite, as well as greenstone, is an igneous rock, the inquiry naturally arises why these two rocks differ so widely in appearance, if indeed they have had the same origin? This is a question which our present knowledge does not enable us to answer with any degree of certainty, nor indeed do geologists profess to do more than offer plausible conjectures to account for these differences.