THE STEIJCTTJEE AND MOTION OF GLACIERS. 
339 
tending to open into fissures, the flattening of the central circle sliowS that the mud is 
longitudinally compressed. On the slope helow ef^ the distortion of the circles intc) 
ellipses is very pronounced; and as the longer axis of each ellipse marlis the line of 
maximum tension, and as the tendency of the mass is to form a fissure at right angles to 
such a line, we should have here, if the substance were not so plastic as to prevent the 
formation of fissures, the state of things observed upon the corresponding portion of the 
glacier ; namely, central fissures perpendicular to the longitudinal axis of the trough, and 
side fissures inclined to the same axis because pointing in the direction of the shorter 
axis of each ellipse. Between gh and iJc the longitudinal tension is changed to compres- 
sion ; the central figure is flattened, while the side ones remain stretched. In the cor- 
responding portion of the glacier we should expect the central fissures formed between 
ef and gh to be squeezed together and closed up, while the lateral ones would remain 
open. This is also the case*. Between ik and mn we have again longitudinal tension, 
and at the corresponding portions of the glacier the transverse central crevasses ought 
to reappear, which they actually do. Below the line corresponding to m%, the widening 
of the valley, in the case now in our recollection, causes the ridges produced at the 
previous slope to break across and form prismatic blocks ; while lower down the valley 
these prisms are converted by the action of sun and rain into shinmg minarets of ice. 
These results appear to be in perfect accordance with those arrived at by Mr. Hopkixs 
on strict mechanical reasoning f. 
We will now seek to show the analogy of slaty cleavage to the laminar structure of 
glacier ice. Eeferring to fig. 8, it will be seen that in the distortion of the side circles 
one diameter is elongated to form the transverse axis of the ellipse, while another is 
compressed to form the conjugate axis. In a substance like mud, as the elongation of 
the major axis continues, its inclination to the axis of the glacier continually changes ; 
but were the substance one of limited extensibility like ice, fissures would be formed 
when the tension had reached a sufficient amount, or in other words, when the major 
axis of the elhpse had assumed a definite inchnation to the axis of the glacier. 
Thus, in a glacier of the form represented by our trough, owing to the swifter motion 
* The possibility of the coexistence of lateral crevasses and compression at the centre may, perhaps, be 
thus rendered manifest : — let ab, cd be two linear elements of a glacier, situated near its side S I. 
> ► 
c d c' d' 
a b d V 
S 1 
Suppose, on passing downward, the line ah becomes shortened by longitudinal pressure to a!V, and cd to c'd', 
which latter has passed a!V on account of its greater distance from the side of the glacier. Taking the 
figure to represent the true change both of dimension and position, it is plam, that though each element 
has been compressed, the differential motion has been such as to distend the liue of particles joining a and d, 
in the ratio If this ratio be more than that which the extensibility of ice can permit of, a side fissure 
wfil be formed. 
t Philosophical Magazine, 1845, vol. xxvi. 
2 T 2 
