Such conditions of enormous pressure we may infer to exist in the deep-seated reservoirs beneath volcanoes, where, besides the weight of the superincumbent rock-masses, we have the compressing force of great quantities of elastic vapour held in confinement. The crystals of which granitic rocks are entirely built up exhibit clear evidence of having been all formed under these conditions of enormous pressure. The glassy base or groundmass of lavas, on the other hand, presents all the characters of materials that have cooled from a state of fusion. Most lavas consist in part of crystals, exhibiting fluid-cavities like those present in granite, and in part of a base, which has evidently been formed by the cooling of a fused mass. We are therefore justified in concluding that the crystals have been formed in subterranean recesses, and that the groundmass or base has consolidated at the surface. The bearing of these conclusions upon some of the great problems presented by volcanoes we shall have occasion to point out in the sequel. [Sidenote: CAUSE OF MOVEMENT OF BUBBLES.] One of the most interesting inquiries suggested by the study of the liquid-cavities in volcanic rocks is that of the cause of the apparently spontaneous movement of the bubbles which we have described as taking place in some of the smaller of them. The ingenious experiments of Mr. Noel Hartley have suggested to Professor Stokes an explanation which is probably the true one. It appears that these minute globes of vapour are in such a state of unstable equilibrium as to be affected by the smallest changes of temperature, and that the variations in the heat of the atmosphere, due to currents of air and the movement of warm or cold bodies through it, are sufficient to cause the oscillation of these sensitively poised bubbles. The short account which we have been able to give in the foregoing pages of the researches that have been carried on concerning the nature of the materials ejected from volcanoes will serve to show that these investigations have already made known many facts of great interest, and that the farther pursuit of them is full of the highest promise. To the scientific worker no subject is too vast for his research, no object so minute as to be unworthy of his most patient study. In some of our future inquiries concerning the nature of volcanic action, we shall be led to an investigation of the phenomena displayed in the sun, moon, comets and other great bodies of the universe; but another road to truths of the same grandeur and importance is found, as we have seen, in an examination of the mode of development of crystallites, and a study of the materials contained in the microscopic cavities of the minutest crystals. CHAPTER IV. THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS.