ANALOGY OF GLACIERS TO LAVA STREAMS. 
153 
become convex as their viscosity increases. Nevertheless, I have seen portions of 
well-bounded streams decidedly convex. 
The appearance of the termination of a lava stream approaches strikingly that of 
a glacier. But this is much more than a vague analogy, and the accounts of faithful 
eye-witnesses prove the resisted motion of the doughy stream to be such as I antici- 
pated. We find it explicitly stated over and over again in the writings of Dolomieu* * * § 
and Della Torre -f~ (and more particularly by the latter), that when a lava stream 
meets with any obstacle in front which checks its course, or when its course is 
checked by its own sluggishness, the stream swells, and gains gradually in thickness 
by the fluid pressure from behind urging its particles forwards and upwards. So 
striking was. this natural effect of semi-fluid pressure, that these old observers attri- 
buted it to a peculiar force developed in the lava, of the nature of “ fermentation,” 
producing intumescence, the only way by which they could account for the vertical 
rise of the fluid, although it was very evident that the result was only what might be 
expected from the nature of the lava. It was also observed that when the lava stream 
had thus attained a certain height, it began to move on again, the necessary result 
of the increased hydrostatic pressure, although attributed by the authors named to 
the heat developed by chemical action. The tenacity with which the idea was long 
adhered to, that the residual fluidity of a nearly cooled lava stream was insufficient 
to account for its progress, without attributing to it the qualities of a second volcanic 
focus, are curious proofs of how long a palpable cause may be rejected as insufficient 
to explain a phenomenon, and a totally imaginary one superadded^. 
I may add, that lava streams sometimes push their extremities up hill glaciers 
do the same. 
In addition to the considerations already stated, which illustrate the viscous theory 
of glaciers, I am glad to avail myself of two which have reached me from indepen- 
dent and impartial sources. 
The first is by Mr. Darwin, who in a small book on “ Volcanic Islands,” published 
about the time that I was engaged in making the preceding observations on Etna and 
Vesuvius, pointed out in a very clear manner the explanation which the veined struc- 
ture of glaciers lends to that of volcanic rocks belonging to the Trachytic and 
Obsidian Series, where the lamination, instead of being obscure and rare, as it 
generally is in the Augitic lavas, owing perhaps to their greater fluidity, and more 
viscid and homogeneous texture, is the general rule. “ The most probable expla- 
nation,” says Mr. Darwin, “ of the laminated structure of these felspathic rocks 
• * Papers in the Journal de Physique. 
t Histoire du Vesuve. Naples, 1771, 8vo, p. 207-9, and several other places. 
1 See the view of the termination of a lava stream in Auldjo’s Sketches of Vesuvius, facing p. 92. The 
reader may also compare the view of a grotto in the lava, in the same work, with that of the source of the 
Arveiron, in my Travels, p. 387. 
§ Hamilton, Campi Phlegraei, folio, vol. i. p. 40, note. 
