species over the area, and that during this period one foot of shale was accumulated, we have the years consumed in depositing the strata of this coalfield, 3000 to 5000 ft. thick; = 15,000 to 25,000 years. But several layers of the shells lie in the thickness of one foot. In the same coal-field, near the base, we find one or more thin beds of shale and calcareous balls, abounding in Goniatites Listeri, and some other marine shells. These Goniatites are of every magnitude, from the youngest not larger than mustard-seed, to specimens three inches in diameter. This fact leads to an inference of the same order, and indicates a period only to be expressed in tens of thousands of years.

These inferences apply only to the rate of accumulation of shales; we cannot adopt the same estimates for the sandstones, which may have been more rapidly, or for the coal-beds which may have been more slowly aggregated. For these latter deposits, fortunately, Liebig and modern agricultural Chemistry have supplied a different and independent basis of computation.

It is now very generally admitted that our coalstrata are derived from the accumulation of trees, and other kinds of plants, on or very near to the places of their growth; at all events the greater part can have been moved but small distances. Plants after their

germination draw from the atmosphere the Carbon which is fixed in their tissues, and combined with Oxygen, Hydrogen, and in some cases Nitrogen, constitutes the main part of their substance. The chemical composition of the atmosphere is everywhere nearly the same; the stimulus of light which is the great determinant of the chemical processes in plants, and which specially governs the absorption of carbonic acid by them, acts with much equality on the various races of plants. Hence much equality in the amount of Carbon taken from the atmosphere, and fixed in the substance of plants in a given time. Liebig has found that the annual product of Carbon fixed in plants so different as Firwood, Hay, Beet-roots, and Straw, is the same in weight, for the same extent of ground. This quantity is about 1000 Hessian lbs. for 40,000 square feet of surface'; in English weight and measure 10 lbs. for 244 square feet.

Supposing the whole of this quantity to be stored up year by year, and converted to Anthracite, the variety of Coal richest in Carbon, such as occurs in South Wales and North America, it would amount to about one inch in 170 years. If converted to ordinary Coal, with about 75 per cent. of Carbon, it would yield one inch in 1275 years. In South Wales 1 Organic Chemistry, 1840.

the total thickness of Coal, some beds being Anthracitic and some ordinary Coal, is 120 feet. To produce this thickness of ordinary Coal, by vegetables growing on the spot, would require 120 × 12 × 127′5 = 183,600 years. The same thickness of Anthracitic Coal would require 120 × 12 × 170 = 244,800 years. Add to either of these periods the lowest number of years found for the deposition of the sediments in the same coal-field, 333,000, and we have as the probable length of time required for the production of the Strata of Coal, Sandstone, Shale, and Ironstone in South Wales, half a million of years.

If now we turn to the Leibnitzian Theory, and apply it to the same problems, we must first settle the limits of the atmospheric power to waste the surface in the early geological periods. This limit at the origin of the oceans needs not to be inquired into; there would indeed be a mighty power in action-perpetually falling floods-the fountains of heaven-wasting a hot and friable surface1. Coming down to the base of the Paleozoic System, and adopting as high a limit for the temperature of the surface of the globe as might be possible to suit the races of Mollusca and Crustacea then coming into view, we may allow 20° higher mean temperature than at present. From this point it must be 1 Babbage. Ninth Bridgewater Treatise.

supposed to have declined to the actual condition, and with the depression of temperature the atmospheric wasting power to have declined also, and in a more rapid progression. For the quantity of moisture sustained in air of different temperatures freely in contact with water diminishes more rapidly than the temperatures'; so that if we take 56° for the present mean temperature of the surface of the earth, and suppose it to have been formerly 76°, the quantity of moisture held up by the atmosphere formerly may be taken at nearly double that now supported in it.

This being the case, and the causes of rain and vicissitudes of seasons the same in kind, we may admit the atmospheric power in wasting the earth's surface to have been double what it is now, and that from that time to the present it has sunk in geometrical progression. Under these conditions the time consumed in the production of the same total effect as that already stated, under the actual powers of the atmosphere, could not be so little as two-thirds of that computed on the hypothesis of uniform action, viz. 63,936,000 years.

But according to the same hypothesis the action of the sea and atmosphere in early times must have been more effective than at present, because of the 1 It varies in geometrical proportion to the temperature.

greater frequency, or greater violence, of movements in the crust of the earth: for thus it would yield more easily to the assaults of the waves and atmospheric vicissitudes. During the earlier periods, land rising through the waves was subject to more rapid erosion by the sea and the atmosphere; short æras of enormous waste productive of conglomerates, longer periods of strong action yielding large masses of sandstone, still longer periods of more equable decay producing more extended beds of clay since indurated to shale and slate.

During the same periods land was sunk in one tract as well as raised in another, perhaps to as great a depth here as the elevation there; while it was sinking, fresh edges were exposed to the sea, and thus the waste was quickened. How are these effects to be calculated? Strictly they cannot be. But, as an illustration, let us suppose that by these operations the resisting power of the rocky masses of the earth's crust was weaker than at present—only one-half as great—so that combining this ratio to resist with that already allowed for the power to destroy, we shall have the earlier atmospheric waste effectively four times as great as at present. This allowed, we shall find the whole time, as given by the Uniformitarian hypothesis, cannot be reduced to so little as, or 38,000,000 years.