The calamity which befell Martinique and St. Vincent will unquestionably lead to a fresh discussion of the causes of volcanic disturbance. Not all of the phenomena involved therein are yet fully understood, and concerning some of them there are perceptible differences of opinion among experts. On at least one point, however, there is general agreement. At a depth of about thirty miles the internal heat of the earth is probably great enough to melt every known substance. Confinement may keep in a rigid condition the material which lies beneath the solid crust, but if an avenue of escape is once opened the stuff would soften and ooze upward. There is a growing tendency, moreover, to recognize the importance of gravitation in producing eruptions. The weight of several miles of rock is almost inconceivable, and it certainly ought to compel "potentially plastic" matter to rise through any crevice that might be newly formed. Russell, Gilbert and some other authorities regard this as the chief mechanical agent in an eruption, at least when there is a considerable outpouring of lava. As to the extent to which water operates there is some lack of harmony among volcanists. Shaler, Milne and others hold that substance largely, if not entirely, responsible for the trouble. They point to the fact that many volcanoes are situated near the coast of continents or on islands, where leakage from the ocean may possibly occur. Russell, on the other hand, regards water not as the initial factor, but as an occasional, though important, reinforcement. He suspects that when the molten rock has risen to a considerable distance it encounters that fluid, perhaps in a succession of pockets, and that steam is then suddenly generated. The explosive effects which ensue are of two kinds. By the expansion of the moisture which some of the lava contains the latter is reduced to a state of powder, and thus originate the enormous clouds of fine dust which are ejected. Shocks of greater or less violence are also produced. The less severe ones no doubt sound like the discharge of artillery and give rise to tremors in the immediate vicinity. In extreme cases enough force is developed to rend the walls of the volcano itself. Russell attributes the blowing up of Krakatoa to steam. The culminating episode of the Pelee eruption, though not resulting so disastrously to the mountain, would seem to be due to the same immediate cause. To this particular explosion, too, it seems safe to assign the upheaval which excited a tidal wave.