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.
IMPERFECTLY FLUID LAVAS.
This lava is an obsidian—that is to say, it is of the add type and completely glassy—but its liquidity must have been very imperfect, seeing that the stream has come to a standstill before reaching the bottom of a steep slope of about 35°. In [fig. 19] there is given a side view of the same stream of obsidian, from which it will be seen that it has flowed slowly down a steep slope and heaped itself up at the bottom, as its fluidity was not complete enough to enable it to move on a slighter incline. An examination of the interior of such imperfectly fluid lavas affords fresh proofs of the slow and tortuous movements of the mass. Everywhere we find that the bands of crystallites and sphærulites are, by the movement of the mass, folded and crumpled and puckered in the most remarkable manner, as is illustrated in figs. [20] and [21]. Similar appearances occur again and again among the vitreous and semi-vitreous acid lavas of Hungary.
Fig. 19.—Lava-stream (obsidian) in the Island of Vulcano showing the imperfect liquidity of the mass.
Fig. 20.—Interior of a Rhyolitic Lava-stream in the Island of Lipari, showing broad sigmoidal folds produced by the slow movements of the mass.
Fig. 21.—Interior of a Rhyolitic Lava-stream in the Island of Lipari, showing the complicated crumplings and puckerings produced by the slow movements of the mass.
RATE OF MOVEMENT OF VESUVIAN LAVAS.