It is not easy to comprehend the conditions under which large masses of molten material have been injected into the crust of the earth. The two main factors in volcanic action--terrestrial contraction and the energy of the vapours in the magma--have no doubt played the chief part in the process. But the relative share of each and the way in which the enormous load of overlying rock has been overcome are not readily intelligible. Let us first consider for a moment the pressure of the superincumbent crust under which the injection in many cases took place. The Whin Sill of England may serve as a good illustration of the difficulties of the problem. This notable mass of intrusive rock has been forced between the stratification planes of the Carboniferous Limestone series in one, or sometimes more than one, sheet. It stretches for a horizontal distance of not less than 80 miles with an average thickness of between 80 and 100 feet. From the area over which it can be traced its total extent underground must be at least 400 square miles (see Chapter xxix.). In any single section the Whin Sill might be supposed to be a truly interstratified sheet, so evenly does it seem to be intercalated between the sedimentary strata. But here and there it diverges upward or downward in such a way as to prove it to be really a vast injected sheet. The age of the injection cannot be precisely fixed. It must be later than the Carboniferous Limestone. There is no trace of any stratigraphical break in the Carboniferous system of the region traversed by the sill. If the injection took place during the Carboniferous period, it does not appear to have been attended with any local disturbance, such as we might suppose would have been likely to accompany the extravasation of so enormous a mass of igneous material. If the date of injection be assigned to the next volcanic episode in the geological history of Britain--that of the Permian period--it will follow that the Whin Sill was intruded into its present position under the superincumbent weight of the whole of the Carboniferous system higher than the platform followed by the injected rock. The overlying body of strata would thus exceed 5000 feet in thickness, or in round numbers would amount at least to an English mile. The pressure of this mass of superincumbent material, at the depth at which the injected magma was forced between the strata, must have been so gigantic that it is difficult to believe that the energy of the magma would have been able to achieve of itself so stupendous a task as the formation of the Great Whin Sill.