ANALOGY OF GLACIERS TO LAVA STREAMS. 
151 
In the upper part of this Fossa the lava has a distinct linear structure where broken, 
in shells parallel to the sides, whose thickness varies from one-third of an inch upwards. 
The position of these surfaces of dislocation is indicated (for illustration) in figure 5 
of Plate IV. 
2. In the vast lava wastes of Etna, we encounter not only a greater extent of 
surface, but a greater variety of condition as to cohesion of the lava streams, and the 
slope down which it has descended, and thus we have a better chance of meeting 
with specimens of the manner in which the semi-solid crust of a lava stream is torn 
up and crevassed by the effect of gravity compelling it into the circumstances of fluid 
motion. From this tendency of all lavas to form slags, and of these slags to be 
splintered, tossed, and remoulded by the action of the still liquid portion of the stream 
below or around, not one-thousandth of the surface bears marks of the simple con- 
dition of fluidity under which it was originally moulded ; and though when viewed 
from a distance, and in connexion with the form of the ground over which it has 
passed, we see plainly enough that it has flowed like a stream, the absence of any 
trace of easy undulating forms which characterise fluids or plastic masses, give to the 
sciarre of Etna (the cheires of Auvergne) an appearance far more removed from pris- 
tine fluidity than the glacier masses of Switzerland. 
In traversing many miles of lava wastes between Nicolosi and Zafarana, on the 
eastern slope of Etna, I met with one singularly favourable specimen of a branch of 
a stream consolidated exactly as it had moved, and undisturbed afterwards. It is 
the part of the current of 1763, called Lava delle Cerve. The branch stream in ques- 
tion may be ten yards wide, and presents a thin crust, which has floated on the viscid 
lava below^, and which, while yet imperfectly solidified, has been urged to move with 
the rest of the stream, and has undergone a process of division and rending accord- 
ingly. The stream has flowed in the direction from left to right in figure 6. The 
lateral parts PP, QQ have been literally torn to pieces longitudinally (as I wrote on 
my note-book on the spot) by the multiplied rents which showed the dislocation of 
the quicker moving central from the lateral parts, and these rents inclined towards 
the centre of the stream in the direction in which it moved. The length of the stream 
w r as divided by transverse rents strikingly convex towards the origin of the stream, 
as shown in the same figure. These cracks were marked by another peculiarity ; the 
cake of floating scoria had not only been cracked across but pushed upwards, generally 
forwards and upwards, before it was finally included in the cooling mass of the 
stream ; the result was the arrangement shown in the longitudinal section, fig. 7, 
which it will be seen resembles the tiling of a house, only that the fractured parts 
do not always overlap, but the anterior edge is tilted upwards. It w r ill thus be seen 
that this tendency to separation acts also in the vertical plane, and the dotted lines 
aa 1 , bb 1 , &c. indicate the direction of its action, coinciding with the surfaces of differ- 
ential motion, which produce what I have called the frontal dip of the veined struc- 
ture of the ice of glaciers. 
MDCCCXLVI. 
X 
