Imatges de pàgina
PDF
EPUB

half-naked peak, chiefly of white granite, shooting 300 feet above the surrounding country. This 1 represented below, (Fig. 17,) as seen on its northside, along which the road passes. The prevailin the vicinity is gneiss; but in this elevation it y hornblende slate, traversed by an enormous vein, a, and exhibiting at least two protruding b, and c, of granite. The vein varies from one our rods in thickness, and the mass b, is four or = across: c, is only ten feet wide. The general of the lamina of the slate is north and south, and rom 15° to 20° east: but we have here the most marks of its having been irregularly upheaved, urbed by the protruding granite. Near the foot ill, the slate is bent upwards, so that the chord of e is several rods long. But it is a curious fact, axis of the elevating force seems not to have cowith the direction in which the vein was erupted. highest point of the curve of elevation, near the he hill, is to the right of the vein at h; and as we he hill we find the slate curved upwards near the re and more, as is shown by the drawing. Indeed, ite of the vein seems to lie on the elevated edges ate; so that the lower side of the vein dips northS ; and does not cut the slate perpendicularly. These uld seem to evince, that the vein made its way the slate, not along the line of the greatest presson the north side of it; probably because there the lded most readily. We may suppose the melted below to have gradually elevated the slate, until a it burst its way laterally through the rock. Such believe, do sometimes occur in existing volcanoes. masses of granite b, and c, are probably other exin which the molten matter bursts its way laterally the slate. And it is an interesting fact in regard ass b, that in some places it still projects over the veral feet, forming in fact an overlaying mass. Inof this kind I have rarely met with in the granite England." Page 480.

mon I

whole

high.

granit

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

[blocks in formation]
[merged small][graphic]

The

on mica-slate: b, is a stratum of amphibolic slate. hole surface exhibited is fifteen feet long and eight feet gh. Through this ledge runs a vein of fine grained anite a foot wide.

"The object of giving this sketch," says Prof. Hitchck, "is to show that this vein has produced no derangeent of the mica-slate: for the different particles of that k occupy the same relative position on the different es of the vein. Hence the vein was introduced subseently to the consolidation of the slate; and probably it s injected into an open fissure."

PASSAGE OF GRANITE INTO BASALT.

Dr. Hibbert describes the manner in which granite has dually passed into basalt in one of the Shetland islands. e basalt extends from the island of Mickle Voe northrds to Roeness Voe, a distance of twelve miles. On west of this there is a considerable mass of granite, the transition from the one into the other is thus debed. "Not far from the junction we may find, disperthrough the basalt, many minute particles of quartz. is is the first indication of an approaching change in the re of the rock. In again tracing it still nearer the nite, we find the particles of quartz dispersed through basalt, becoming still more numerous and larger, an ease of magnitude even extending to every other deption of particles. The rock may now be observed

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

We see, from these examples, that granite has been ced from below into the fissures of other rocks which ere superincumbent, consequently, which were deposited er the granite was formed. In several instances it may observed also, that the granite does not reach the surce, by which it is proved that these veins could not have tered from above, a theory long maintained by those ho claimed that granite was of aqueous origin. Besides,

e indications of fusion which these veins present, the pasge of granite into basalt, a rock which all agree bears e marks of fire, is additional evidence that they had a -inmon origin.

But if we consider granite veins to have forced their ay from below, in a state of igneous fusion, then we ight expect, that when the mass came into contact with ratified rocks, the strata would be separated, and that the id matter would run between them, at least to a short stance, and especially near the surface, where the presre would present little resistance to the separation of the rata. Now this is precisely what is known to have hapned in numerous instances, one of the most striking exmples of which occurs at Glen Tilt, in the Grampian ountains in Scotland.

The

the ap

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

ity," sa

to retic

toward per, on pear e visibly

on the

midst

0

ous scl

The

blue, an

mates

by the

it assu

This ch intrudin

Thes

to show ance of

authors.

[blocks in formation]
[merged small][graphic]

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

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

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

GRANITE OF DIFFERENT AGES.

All the older geological writers believed that granite

Litiva rock of our globe and the one on which

[ocr errors]

thers reposed. They also considered this rock as where of a similar age, the idea of successive formaof granite having never until recently been advanced. e opinions were founded on the general facts, that this lies beneath all others, and that it contains no organic ins, which facts even at the present day we must owledge to be generally true. More extensive obserns have, however, shown many exceptions to these there having been discovered instances where granot only penetrates through, and reposes on stratified , but also where the rocks invaded by it contain orremains. Thus Dr. Macculloch describes a conside mass of granite in the Isle of Sky, which is incumon limestone, and shale. The limestone at some disfrom the granite contains shells, but in its immediate ty, no shells appear, the limestone being converted pure crystalline marble. This change, as well as the uction of the shells, is attributed to the heat of the te at the time of its protrusion.

different part of the Alps, similar phenomena occur, e, according to the observations of Beaumont, and s, granite is seen penetrating through secondary strata, contain belemnites, and other fossil organic remains. Norway, also, Von Bush discovered a mass of granite aying a bed of secondary limestone, containing a vaof fossil shells.

spar, crysta

ese and other instances of the kind, must however be dered as exceptions to a general rule, there being no , but the granite which universally forms the deeprtions of the crust of our globe, is the eldest of our

minut

of gra

inches

[blocks in formation]

DIFFERENCE BETWEEN IGNEOUS ROCKS.

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

ences.

ture, and

Next in The term

erence to

are above

« AnteriorContinua »