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. [Illustration: Fig. 19.--Lava-stream (obsidian) in the Island of Vulcano showing the imperfect liquidity of the mass.] [Illustration: 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.] [Illustration: 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.] [Sidenote: RATE OF MOVEMENT OF VESUVIAN LAVAS.] But, although the temperature of lava-streams and the fusibility of their materials may in some cases account for their condition of either perfect liquidity or viscidity, it is clear that in other instances there must be some other cause for this difference. Thus it has been found that at Vesuvius the lavas erupted in modern times have all a striking similarity to one another in chemical composition, in the minerals which they contain, and in their structure. They are all basic lavas, which when examined by the microscope are seen to consist of a more or less glassy magma, in the midst of which numerous crystals of augite, leucite, olivine, magnetite, and other minerals are scattered. Yet nothing can be more strikingly different than the behaviour of the lavas poured out from Vesuvius at various periods. In some cases the lava appears to be in such a perfectly liquid condition that, issuing from the crater, it has been described as rushing down the slope of the cone like a stream of water, and such exceedingly liquid lavas have in some cases flowed to the distance of several miles from the base of the mountain in a very short time. But other Vesuvian lavas have been in such a viscid condition that their rate of movement has been so extremely slow as to be almost imperceptible. Such lava-streams have continued in movement during many years, but the progress has been so slow (often only a few inches in a day) that it could only be proved by means of careful measurements.