When the observer is near the source of the lava, he sees matter in a state of fusion, which, like a torrent of liquid fire, runs along, with more or less impetuosity, between two banks formed by itself. But as soon as the surface of the torrent cools to the point of congelation, it loses the splendour of its first incandescence. The part which begins to harden breaks readily in some lavas into fragments which float on the viscous fluid beneath; these, increasing in number with distance from the source, conceal the molten matter beneath and retard its progress, and at last nothing is seen but the more or less red-hot scoriæ moving along. These lavas I shall call "_Lavas with fragmentary scoriæ_."
On other occasions, a skin forms on the surface of the lava, which, gradually thickening, keeps flexible for some time, and then wrinkles or swells or extends and breaks to give egress to the hot fluid within, which, in its turn, skins over and repeats the same phenomena. This I shall call "_Lavas with a united surface_."
These, in their course, discharge less smoke than the first, draw out more easily into threads, and, when cold, have a dark colour, something like bitumen or pitch. _The lava with fragmentary scoriæ_, when stretched, breaks easily, discharges smoke copiously, and, when hardened, has a more bluish tint, like clods of upturned earth (_formato di zolle_). It is noisy in its course, because the incoherent scoriæ that it carries along strike and crunch against each other; the other lava flows silently, except for a sort of crackling arising from the actual fracturing up of the solid skin by distension from the liquid matter within. If required to give the mineralogical characteristics of this lava, I would say that it was rich in leucite and contained little or no pyroxene; the fragmentary lava, on the contrary, is poor in leucite and rich in pyroxene. The lavas of 1871 were of the "united surface" character; those of 1872 were "fragmentary," with some characteristics which I shall describe:
1. They were of the clearest tint I have ever seen, when regarded superficially, but, when broken, the fracture was darker than any other lava.
2. They had very little leucite and abounded in pyroxene and olivine, and sometimes contained a few crystals of amphibole.