The volume of injected material is likewise deserving of special attention. Many sills exceed 300 or 400 feet in thickness; and some laccolites must enormously surpass these limits. The intrusion of so vast a body of new material into the terrestrial crust will necessitate either a corresponding elevation of that part of the crust overlying the injected magma or a subsidence of that part underlying it, or some combination of both movements. It is conceivable that, where the body of protruded magma was large and the thickness of overlying crust was small, the expansive force of the vapours under high tension in the molten rock may have sufficed for the uplift. This result will be most likely to be effected around a volcanic chimney where the magma has the least amount of overlying load, and encounters that relief from pressure which enables it to become a powerful agent in terrestrial physics. But in the case of the larger bodies of injected rock, especially where they do not seem to have been accompanied by the opening of any volcanic vents, the propulsion of the igneous material into the crust has probably been effected as a consequence of disturbance of the terrestrial crust. When the strain of contraction leads to the pushing upward of the terrestrial areas intervening between wide regions of subsidence, even though the differential movement may be slight, the isogeotherms undergo deformation. The intensely hot nucleus is squeezed upward, and if in the process of compression ruptures take place in the crust, and cavities in it are consequently opened, the magma will at once be forced into them. Such ruptures may be expected to take place along lines of weakness. Rocks will split along their stratification-planes, and the tendency to separation along these lines may be aided by the readiness of the energetic magma to find its way into and to enlarge every available opening. Hence we may expect that, besides vertical fractures, leading to the production of dykes and bosses, there will often be horizontal thrusts and ruptures, which will give rise to the formation of sills.