There can be no doubt that the temperature of lavas varies greatly in different cases. This is shown by the fact that while some lavas are in a state of complete fusion, similar to that of the slags of furnaces, and like the latter, such lavas on cooling form a glassy mass, others consist of a liquid magma in which a larger or smaller number of crystals are found floating. In these latter cases the temperature of the magma must be below the fusing-point of the minerals which exist in a crystalline condition in its midst. It has indeed been suggested that the whole of the crystals in lavas are formed during the cooling down of a completely fused mass; but no one can imagine that the enclosed crystals of quartz, felspar, leucite, olivine, &c., have been so formed, such crystals being sometimes more than an inch in diameter. The microscopic examination of lavas usually enables us to discriminate between those complete crystals which have been formed at great depths and carried up to the surface, and the minute crystalline particles and microliths which have been developed in the glassy mass during cooling. Crystals of the former class, indeed, exhibit abundant evidence, in their liquid cavities and other peculiarities, that they have not been formed by simple cooling from a state of fusion, but under the combined action of heat, the presence of water and various gases, and intense pressure. As we have already seen, the different lavas vary greatly in their degrees of fusibility. The basic lavas, containing a low percentage of silica, are much more fusible than the acid lavas, which contain a high percentage of silica. When the basic lavas are reduced to a complete state of fusion their liquidity is sometimes very perfect, as is the case at Kilauea in Hawaii, where the lava is thrown up into jets and fountains, falling in minute drops, and being drawn out into fine glassy threads. On the other hand, the less fusible acid lavas appear to be usually only reduced to the viscous or pasty condition, which artificial glasses assume long before their complete fusion. Of this fact I have found many proofs in the Lipari Islands, where such glassy, acid lavas abound. In fig. 6 (page 43) a lava-stream is represented on the side of the cone of Vulcano. [Sidenote: IMPERFECTLY FLUID LAVAS.]