206 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
The “ ductility” is indeed not great ; the compact ice even of the slowest moving 
glaciers bears evidence, in the veined structure, or “ blue bands,” to the bruise which 
it has received from the all-powerful strain which has acted on it. When the differ- 
ence of motions is excessive, or the slope occasions the speed to be greater than per- 
mits the gradual molecular adaptation of the semi-rigid parts to one another, the 
masses are broken up and fall more or less tumultuously; the strain being then re- 
moved by the dislocation, the veined or bruised structure is invariably extinguished at 
last. I shall quote a series of examples of the gradation of phenomena, which I con- 
ceive to be plainly connected by a common cause. 
1. In any torrential glacier, such as the Glacier des Bossons, the upper part of the 
glaciers of La Brenva, Allalein, or the Rhone, and many others, the fractures are so 
numerous that the ice descends in blocks, almost as water in a cascade often does in 
spray, and hence the internal strains being destroyed, no structure is developed, or 
if previously developed, tends to wear out*. 
2. In a glacier moving torrentially, that is with frequent and considerable changes 
of velocity, but without being divided into blocks by intersecting crevasses, we find 
real internal cracks in the ice, some feet in length, and an inch or more in thickness, 
marked by the pure frozen water which fills these spaces in the comparatively opake 
whitish ice of which glaciers descending rapidly from the region of the ndvd are com- 
posed. Such are peculiarly visible in the lower and more accessible region of the glacier 
of Bessons^ ; perhaps the most instructive which can be named as showing these 
infiltrated cracks, which by their dimensions, direction, and in every other particular, 
form a true link between the longitudinal dislocations of a torrential glacier, and the 
perfect veined structure or bruise into which it passes by imperceptible gradations, 
including a perfectly regular development of the frontal dip, where we might expect 
it to be well shown, for the observations of page 184 show that the lowest portion of 
winter, a hollow iron shell be filled with water and exposed to the frost with the fuze-hole uppermost, a por- 
tion of the water expands in freezing, so as to protrude a cylinder of ice from the fuze-hole ; but if the ex- 
periment be continued, the cylinder continues to grow, inch by inch, in proportion as the central nucleus of 
water freezes. “ In the first instance,” says Mr. Christie, “ a shell of ice containing water was formed, no 
doubt, within the iron shell, and the fuze-hole might be filled by the expansion of the water in the act of 
freezing ; so that there may be no reason for attributing plasticity to the ice as far as this goes ; hut the shell 
of ice once formed, and the fuze-hole filled with ice, the subsequent rise of the ice must have proceeded from 
the ice of the interior shell being squeezed through the narrow orifice. No thawing took place during the 
process. Does not this show plasticity even in very small masses of ice ?” I have also been lately informed, 
on excellent authority, that in a new work by a most eminent German mineralogist, the plastic character of 
ice in masses is assumed as an admitted fact. In corroboration of what has been said in the text, I may farther 
add, that whilst these sheets are passing through the press, I observe in the Athenaeum (June 20, 1846), an 
account of a patent process for moulding solid tin into tubes and other utensils, in the course of which it is 
stated that “ tin under a pressure of about twenty tons to a circular inch, will run according to the law of 
fluids.” 
* See Third Letter on Glaciers. Travels, Appendix, p. 407. 
t See Travels, p. 181. 
