That in the case of most great volcanic mountains, or systems of mountains, vast reservoirs of liquid lava must exist in the earth's crust far below the surface, there can be little room for doubt. Whether such fluid masses are in direct or indirect communication with a great central reservoir, even supposing such to exist, is a totally different question. In many cases the outburst of volcanoes in more or less close proximity has been observed to take place simultaneously, while in others the commencement of the eruption of one volcano has coincided with the lapse into quiescence of another in its vicinity. On the other hand, the remarkable case of the volcanoes of Hawaii seems to indicate that two vents in close proximity may be supplied from perfectly distinct reservoirs of lava. The active craters of Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000 feet respectively above the sea level; yet the former is sometimes in a state of violent activity, with which the latter shows no signs of sympathy whatever. We shall, in a future chapter, adduce evidence that the liquid lavas in underground reservoirs may undergo various stages of change in the enormous periods of time during which habitual volcanic vents are supplied from them. We have already shown that the character assumed by a mass of fused material in cooling varies greatly according as the cooling takes place rapidly at the surface or slowly under enormous pressure. In the former case a glassy base is formed containing a greater or smaller number of crystallites or embryo crystals, in the latter the whole rock is converted into a mass of fully-developed crystals. [Sidenote: CONSOLIDATION OF LAVAS AT GREAT DEPTHS.] The lavas which are poured out at the surface consist, as we have seen, of a glassy magma in which a greater or smaller number of crystals are found which have been borne up from below. The great dykes and intrusive sheets consist for the most part of a mass of small or imperfectly developed crystals in which a number of large and perfectly formed crystals are embedded. Such rocks are said to have a 'porphyritic' structure. The rocks formed by the consolidation of the liquid masses in the underground reservoirs are found to be perfectly crystallised, the crystals impressing one another on every side and making up the whole mass to the exclusion of any paste or magma between them. The crystals in those rocks which have consolidated at these vast depths exhibit evidence, in their enclosed watery solutions and liquefied carbonic acid, of the enormous pressures under which they must have been consolidated. The lavas, the more or less porphyritic rocks of the dykes and sheets, and the perfectly crystalline (granitic) rocks of the underground reservoirs pass into one another, however, by the most insensible gradations.