le, the measure of increment attained during the intervals of eir deposition. These belts contain each their own peculiar Ss of fossils which determine their relative ages. In succesn, the several suites or families of rocks rest upon the inverted tcrop or inclined edges of the older groups. Thus the history organic life upon the globe, the incoming of new races and the tinction of old ones, as contained in these deposits, becomes a ale of measurement of the elevations, disruptions, and everarying conditions of the inorganic crust, while in the inverted, islocated state of the crust itself, we mark the several throes by hich it was lifted above the waters. . Not one of the fossiliferous eds enveloping the granitic and crystalline nucleus of the chain of the Alps but has been shifted out of its original horizontal posiion, and the shift of the subjacent having always preceded the leposition of the overlying formation, it follows that, in addition o the intumescence of the chain, there must have been a series of oscillatory and elevatory movements before attaining its final altitude. But after the consolidation of the whole rocky strata, and while the waters were still many thousands of feet in depth, the superficial accumulations were being deposited the bowlder drift, and erratic blocks, either by icebergs or other causes, were floated into position—and it was not until every one of these traveled stones, fresh even now as when torn from the living rock, were quietly settled down into the bottom of the sea, that Mont Blanc had displayed a moiety of its massive outline, or towered to one-half of its present colossal grandeur. The elevation of Ben-Mac-Dhui dates from the era of the old red sandstone formation. Mont Blanc was invaded on all sides by a sea that received the latest of the tertiary deposits. Both were submerged during the cataclysm which produced the bowlder clay; but as no increment to its bulk was derived from this cause, BenMac-Dhui falls geologically to be reckoned a completed, and therefore a far older, mountain than Mont Blanc, which had not attained its full altitude and bulk until the expiration of the Pleiocene age!

Such are the mighty agencies contemplated by the geologist in the various later changes which have affected the surface of our globe. The rill, the river, the torrent, the glacier, the earthquake,

the volcano, are still in operation, but only as faint images of the enormous powers which in the more ancient times have been ai work. That the earth has been repeatedly encroached upon by the waters every principle of his science goes to establish; but out of every convulsion he sees a better and more stable condition of things to have emerged. If the bowlder drift and the cold plastic clay formation point to a continuance of sunless, lifeless seasons, he forgets not, as the products of the period, that twothirds of the soil of Great Britain and of the grain-bearing lands of the continent, have been derived from these accumulationsthe industrial monuments of their invasion in every quarter of the world.

CHAPTER III.

THICKNESS OF THE EARTH'S CRUST - CENTRAL HEAT.

THE question arises, Since upon geological grounds it is denonstrable that the crust of the earth has been repeatedly upheaved and broken, have we reason to conclude that similar states of paroxysm and convulsion may not again return? This brings us to the consideration of two very interesting problems, namely,—THE THICKNESS OF THE EARTH'S CRUST—And THE DOCTRINE OF CENTRAL HEAT. Have we any means of determining either of these points? The doctrine of the igneous origin of granite and other rocks proceeds upon the assumption of a vast reservoir of heat existing somewhere within the interior; and the question to be solved is--What is the thickness of the solid crust beneath which the molten rocks have their origin? and what the cause of their fusion?

I. An opinion has long prevailed among geologists of a certain school, that the crust of the earth is of very limited dimensions. A thin coating of primary crystalline rock is interposed betwixt the sedimentary strata above, and the intensely incandescent mass of which the interior is composed. The experiments of Fourier establish a formula of increasing temperature of the strata in a descending series, and from the rate of this increase, it is inferred, that about one hundred miles below the surface the entire nucleus is in a state of complete fusion. Some have even assumed the melting point to be less than thirty miles, when "the next contiguous matter is in a state of fusion, at a temperature probably higher than any that man can produce by artificial means, or any natural heat that can exist on the surface."* Sir John Leslie *Dr. Pye Smith on Scripture Geology.

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attempted a demonstration of the ultimate resolution of the materials into light, as the only element capable of resisting the vast pressure of the outer crust; and, erroneously assuming the modulus of compressibility of air, water, the metals, and all known earthy substances to be invariable, however greatly the pressure may be increased, this ingenious philosopher came to the conclusion, that, instead of Tartarean darkness, the offspring of superstition, the inner chambers of the earth are filled with luminous ether, the most pure, concentrated, and resplendent. Darwin believes that much of the vast continent of South America is suspended over an inner sea of liquid fire, and says, that, "daily it is forced home on the mind of the geologist, that nothing, not even the wind that blows, is so unstable as the level of the crust of the earth."

With the fires of Etna and Vesuvius raging on the one side, and the recent though extinct volcanoes of Auvergne and the Cantal seated so near on the other side, what security is there, amidst so many undoubted facts of the mobility of the land, that these vast piles of Alpine mountains may not again, through mere mechanical weight, break through the film of crust on which they rest, and sink into the abyss from which they so lately emerged! The doctrine of central heat, it may be replied, does not necessarily imply the universal fluidity of the central mass, an opinion supported by Lyell, Poisson, and other eminent philosophers; while there is reason to infer, as repeatedly stated, that there is no identity of scale and mechanism between volcanoes now active, and the igneous causes which gave birth to these and other stupendous mountain-chains.

But astronomy gives a different and more comfortable solution of the problem. The influence of the moon alone, it would appear, acting upon our planet, requires a thickness of crust of at least ONE THOUSAND MILES, to prevent the fabric of the globe from being severed into fragments. The earth, considered in connection with its own planetary system, has three distinct motions in space, a fact in science usually illustrated by the movements of the common spinning-top. A more striking illustration may be seen in the steam-vapor which has aided you onward, that living cloud of light and heat which towers and floats away

in these beautifully curling wreaths. Like the trail of the comet, how gracefully it sweeps over the plains in its forward movement: then it turns to the right or left in the direction of the wind: and then, in a third convolution, every globule of the airy mass is twirling on an axis of its own. Equally buoyant is the earth, hung upon nothing, and cleaving the liquid firmament.

on its axis, causing the vicissitude of day and night; it moves through its orbit, making the circuit of the sun and the diversity of the seasons; and, in addition, there is an oscillatory motion like the unsteady zig-zag twistings of the carriage-train, occasioned by the excess of the equatorial over the polar diameter. This excess amounts to about a three-hundredth part. But, small as it is, it exerts an assignable influence over the cohesion or attraction of the solid framework. Now, by a nice mathematical demonstration, resting on the sun and moon's attraction, Mr. Hopkins infers, as indicated by the phenomena of precession and nutation, that the minimum thickness of the earth's crust cannot be less than one-fourth or one-fifth of the earth's radius. The theorem is of too abstract a nature to be here introduced; but it appears from it that the observed amount of precession requires this degree of solid matter, which gives a clear depth of solid arch over either vacuum, resplendent light, or fiery fluid, of from eight hundred to a thousand miles. This may well allay the fears of the most timid as to the stability of the ground beneath his feet, whatever be the state of the interior, or under whatever modifications the materials therein may exist.

II. But if this thickness of crust is required now, it must have been equally required in all past time: hence, it may be argued, no security is thereby afforded against the bursting out of the pent-up fires, or disruption of the outer crust? Now, it has been questioned whether there be such a thing as a CENTRAL fluid heat at all, while the solidity of the earth throughout has been maintained as more in unison with the principles of established science.

The doctrine of a central heat is as old as the days of Bishop Burnet, who imagined that the internal fire, pre-existent in the bowels of the earth, was the agent employed in breaking up the