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.
3. Their specific gravity varied with their porosity; the most
compact attained 2·75.
4. These lavas carried along in their course a quantity of
scoriæ which had long been subjected to the action of the acids
of the fumaroles close to the craters, and also a great many
bombs (_bombe_)--that is, round masses similar to those ejected
from craters. These varied in size, some having a diameter of
four to five meters. They frequently contained a large nucleus
of very leucitic lava, like that of 1871, with a larger or
smaller quantity of feroligiste (peroxide of iron). Others
contained lavas changed by the action of the acid vapours near
the craters. These bombs must have flowed out with the lava,
for they are found through its whole course, and they were
certainly not ejected from the crater; for not only are they
found on the lava exclusively, but masses so enormous were not
thrown up from the craters during the eruption; those lying on
the cone near the craters seldom exceed a decimetre in
diameter.
As to the qualitative chemical analysis of the lavas, it always presents
the same elements, with the exception of small quantities of some
metals, lead for example, which have escaped the researches of good
chemists, but which I have constantly found in the sublimations of the
fumaroles of the lava. With respect to the quantitative analysis, two
specimens of the same lava appear indeed to have their constituents in
different proportions. To arrive at any conclusion a long and patient
investigation, requiring means and assistance which the Observatory does
not possess, would be necessary.
Professor Fuchs, of Heidelberg, has devoted himself to this work for
years past, and if he continue it with well-selected and sufficiently
large specimens we may hope some day to obtain satisfactory results.
5. Every specimen of lava which I examined with a very
sensitive magnetoscope improved by myself, was invariably
magneto-polar, not excepting the pieces of the bombs, whether
rejected from the crater or carried along with the lava.