A little consideration will suffice to convince us that these conclusions are not based upon any good evidence. The plutonic rocks, as we have already seen, exhibit sufficient proofs in their highly crystalline character, and in their cavities containing water, liquefied carbonic acid, and other volatile substances, that they must have been formed by the very slow consolidation of igneous materials under enormous pressure. Such pressures, it is evident, could only exist at great depths beneath the earth's surface. Mr. Sorby and others have endeavoured to calculate what was the actual thickness of rock under which certain granites must have been formed, by measuring the amount of contraction in the liquids which have been imprisoned in the crystals of these rocks. The conclusions arrived at are of a sufficiently startling character. It is inferred that the granites which have been thus examined must have consolidated at depths varying from 30,000 to 80,000 feet beneath the earth's surface. It is true that in arriving at these results certain assumptions have to be made, and to these exception may be taken, but the general conclusion that granitic rocks could only have been formed under such high pressures as exist at great depths beneath the surface, appears to be one which is not open to reasonable doubt. If, then, granites and similar rocks were formed at the depth of some miles, it is evident that they can only have made their appearance at the surface by the removal of the vast thickness of overlying rocks; and the sole agency which we know of that is capable of effecting the removal of such enormous quantities of rock-materials, is denudation. But the agents of denudation--rain and frost, rivers and glaciers, and sea-waves--though producing grand results, yet work exceeding slowly; and almost inconceivably long periods of time must have elapsed before masses of rock several miles in thickness could have been removed, and the subjacent granites and other highly crystalline rocks have been exposed at the surface. [Sidenote: ANCIENT AND MODERN VOLCANIC ROCKS.]