he mica-slate being superincumbent on the gneiss, and his on the granite, and we shall see in another place that his was undoubtedly the case. g is a great bed of quartz, ncluded in the micaceous beds, and being much less ubject to the disintegration by the weather, rises above he mica. 4 4, are beds of clay-slate, or roof-slate, on the utside of the mica-slate. 5, is an overlaying mass of orphyry, resting on the mica and clay-slate. 6, a small ed of mica-slate, resting between the central peaks of ranite, with the strata bent and sloping in opposite diections, forming a dish-like cavity. Above 7 is seen a ed of clay and gravel in strata, lying nearly horizontal n the upright edges of the clay-slate, demonstrating their ubsequent and independent formation.

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

66

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

This is the most distinctly stratified of all the primitive. cks, and it is a singular circumstance, that its strata are mmonly very highly inclined,-sometimes nearly, or

ite vertical This rock is associated with granitic rocks

Primitive Limestone. This is called primitive, to dis-
guish it from the secondary, or that which has been
re recently formed; for limestone is of all ages from
t which is now forming at the mouth of the Rhone, to
t which has the antiquity of the granitic mountains.
Primitive limestone is crystalline in its structure, and
ound associated with granite, gneiss, and mica-slate,
ng often intermixed with the latter, or alternating in
ers with it. No organic remains are found in this rock,
hence, like granite, it is supposed to have been formed
ore the creation of living beings. When white and
e, it is known in the arts, under the name of statuary
rble, of which the finest specimens of ancient as well
nodern sculpture are made. It is found particularly in
y, Switzerland, and the Grecian Archipelago
rara marble is a primitive limestone.

The

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

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and

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hornt the m

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Porphyry derives its name from a Greek word, signifypurple, because the first rock to which this name was lied had a purple color. At present, however, any khaving a compact, or paste-like base, with imbedded stals, is called by this name, whatever its color may be. Porphyry has the appearance of having once been in form of a soft paste, into which crystals of various ds, but most commonly felspar, have, by some unknown ans been introduced. When associated with granite, phyry is considered a primitive rock, but is sometimes ondary, and sometimes volcanic. It may, perhaps, be sidered as the connecting link between granitic rocks, those of igneous origin.

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

It w origin,

were fir

suming

the pres

us that I

known, form unt

indeed a all quest there co

crystal is

that these

to the las

sume cry

The k

is obviou

Some sut others in granite a there is r

the comb

water, or

see, affor

erial, such as the ancients constructed, is much too great nd expensive for the present age.

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

Sienite. This rock is composed of quartz, felspar and ornblende. It may be considered as a granite in which e mica is replaced by hornblende; it, however, somenes contains small portions of mica. Its structure is

anular like that of granite, and its prevailing color is ellowish white, mottled with black, giving it a gray aparance. The city of Boston contains many magnificent lumns of sienite. It is associated with granite, into hich it gradually passes, as the mica takes the place of

e hornblende.

IGNEOUS ORIGIN OF GRANITE.

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

Thus

The kind of fluid in which the particles are dissolved, it bvious, must depend on the kind of substance. e substances are soluble in water, others in acids, and ers in caloric. Now, although the materials composing ite are scarcely soluble by any artificial means, still e is no doubt but under a very high temperature, with combined aid of pressure, they would be soluble in er, or in caloric alone, and the phenomena, as we shall

that the latter was the

t, and that the materials composing granite were once in elted state.

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

The igneous origin of trap rocks has long been acowledged by all competent geologists, but the general eement that granite had the same origin is only of ret date. The proofs however of the origin of both are rly the same.

Jnder the "Origin and phenomena of Trap Rocks," ill be seen that dykes or veins of basalt often protrude ough the strata of other rocks, and that where they e into contact with these strata, the effects of heat are ays apparent. The illustrations by diagrams, also ve that these veins, or dykes, were forced through the ures, or spread between the strata of the rocks, while former was in a soft or semifluid state. The same nomena are found to attend veins of granite which verse other rocks, there being every indication that these s were forced up from below in an ignited and soften

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,

but

the

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

ut infer that they were forced up in a softened state from he underlaying granite with which their trunks are incororated. Similar instances, that is, of granite veins traversng stratified rocks, and also rocks of granite, are known O occur frequently and in various parts of the world. In Curope such cases were formerly considered singular and mportant phenomena, and as they went to prove the ig eous origin of granite, they were described with great rolixity and exactness. But the progress of observation as shown that granitic veins are quite common, and that articularly in mica-slate, examples may be seen in almost my place, where circumstances allow the rock to be exmined a few yards below the surface, and often on the arface itself.

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

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

ce.

ca.

11

ilea distant comarkable