196 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
let a plumb-line down even fifty feet without grazing the sides 1 and to what an in- 
significant fissure has the gaping crevasse dwindled even at that small fraction of the. 
glacier’s thickness! Supposing the crevasse to become uniformly narrower, how 
soon would it be extinct ! 
Again, the crevasses which traverse the surface of the glacier have almost always 
a determinate direction or directions, of which the simplest type seems to be that of 
perpendicularity to the veined structure *, which, generally speaking, occasions a 
convexity of the lines of fissure towards the origin of the glacier. Opposite Montan- 
vert the crevasses form two systems inclined 65° to one another, but this appears 
to be a casual occurrence arising from a fresh strain being imposed on the ice owing 
to its rigidity when the direction of the bed or trough suddenly changes, and the 
two-fold systems probably coexist but for a short space, one tending to close whilst 
the other opens. Be this as it may, unless where a glacier is falling headlong in the 
manner of a cascade, the crevasses do not produce any actual dislocation of its mass 
into blocks or fragments, since the crevasses rarely intersect even where most nume- 
rous, but almost invariably thin out in the solid mass, whilst another crevasse takes 
its origin a little to one side or other, leaving a firm connection of ice between them ; 
and the difficulty and danger of traversing a glacier where much crevassed, does not 
arise from the necessity of leaping from square to square of ice, but from having to 
traverse these bridges of icy communication, which even there link the glacier toge- 
ther, and which are almost always sharp on their upper edge when the season of the 
year is pretty far advanced, owing to the continual dripping. 
The occurrence of crevasses which cut up a glacier into square or trapezoidal 
blocks, is sufficiently infrequent to deserve notice. Such occur when a glacier of the 
second order descends over a boss of granite, or a surface convex in all directions. 
We have then radiating crevasses combined with concentric ones, producing a 
tartan-like appearance. Such may be seen in a glacier of the second order on the 
south side of the Aiguilles of Charmoz and Grepon, above the Glacier du Geant ; 
and it is a very convincing proof of the essential tenacity of a glacier, that, with a 
surface so scarred and intersected, the fragments do not fall away in avalanches. 
This only is to be explained by the consideration that, thin as are the glaciers of the 
second order, the apparent dislocation is only superficial. 
Were the inequality of the central and lateral movement of the glacier mass to be 
attributed to longitudinal fissures or discontinuities, by means of which broad stripes 
of ice slide past each other, we should have to demonstrate the existence of such 
fissures, which could not be always close unless either (1) the surfaces were mathe- 
matically adapted to slide over one another, or (2) the ice possessed sufficient plas- 
ticity to mould the surfaces to one another’s asperities, in which case the plasticity 
would alone be sufficient without the discontinuity to explain the motion of the ice. 
These longitudinal fissures, cutting the common transverse fissures perpendicularly, 
* Travels in the Alps, p. 171. 
