152 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
3. At no great distance from this lava, and near the foot of the hillock called the 
Serra Pizzuta, between the last-named point and the valley of Tripodo, I observed a 
transverse section of a lava stream exhibiting an arrangement in bands or plates, 
nearly parallel to the side of the current, but inclining towards the centre. 
4. Between Zafarana and the Porta Calanna (Etna), a remarkably pretty illus- 
tration occurs in the surface of an old lava stream, worn and polished by the 
action of a brook. Where the lava has had to turn an abrupt corner of a rock, A, 
figure 8 (which represents a ground plan), the progress of the lava being violently 
checked by the resistance of the projecting mass, has been torn up into longitudinal 
shreds, which from imperfect fluidity have not reunited, but have left open cavities 
of the form represented in the figure, which exhibit with remarkable fidelity the forms 
of the fissures with which glaciers are sometimes traversed, when they are subjected 
to sudden transitions in their states of motion (as in the glacier des Bossons at Cha- 
mouni), and which coincide in direction with the veined structure, and pass into it 
by imperceptible gradations. 
5. What I have called the frontal dip of the veined structure in glaciers*, I have 
explained by the accumulation of a sluggish mass of considerable extent upon a floor 
or bed offering the resistance of intense friction ; in consequence of which the mass 
of ice, urged downwards and forwards by its intense weight, being resisted by the 
friction of that which immediately precedes it, must yield in the direction of least 
resistance, or squeeze itself in a slanting direction forwards and upwards , and thus 
sliding over the resisting mass immediately in front, will produce surfaces of discon- 
tinuity or differential velocity in that direction. Such a result I inferred from general 
principles without reference to any particular example, and the explanation of the 
superficial convexity of the lower part of many glaciers was evidently satisfactorily 
explained by it. 
The convex swelling form of a viscous stream will depend principally upon the 
relative measure of two quantities, the stiffness or viscosity of the fluid, and the in- 
clination of the surface ; although it will also depend on the part of the stream, 
whether near the origin or the termination, which we consider. 
I have found this variation from concave to convex, depending upon circumstances, 
alike in glaciers and lava streams. Some very highly inclined small glaciers existing 
at considerable heights, and therefore very hard and consistent, are, nevertheless, 
deeply concave from end to end, the slope compensating for the stiffness of the 
matter ; such is a beautiful glacier, named, as far as I can learn, La Gria, or Glacier 
de Bourget, which descends from the Aiguille de Goute towards the valley of Cha- 
mouni. See Plate IV. fig. 9. 
Many, perhaps most, lava streams, where they have well-determined banks, are 
concave during the longer part of their course, but towards their termination they 
* See my Travels in the Alps, 1st Edit. pp. 167, 376, and letter to Dr. Whewell in Jameson’s Journal, 
Oct. 1844. 
