ON THE STRUCTURE AND MOTION OF GLACIERS 17 
lateral ones would remain open. This is also the case.! Between 
zk and mn we have again longitudinal tension, and at the correspond- 
ing portions of the glacier the transverse central crevasses ought 
to reappear, which they actually do. Below the line corresponding to 
mn, 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 shining minarets of ice. 
These results appear to be in perfect accordance with those arrived at 
by Mr. Hopkins on strict mechanical reasoning.” 
We will now seek to show the analogy of slaty cleavage to the 
laminar structure of glacier ice. Referring 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 extensi- 
bility like ice, fissures would be formed when the tension had reached 
a sufficient amount, or in other words, when the major axis of the 
ellipse had assumed a definite inclination to the axis of the glacier. 
Thus, in a glacier of the form represented by our trough owing to 
the swifter motion of the centre, we have a line of maximum pressure 
oblique to the wall of the glacier, and a line of maximum tension 
perpendicular to the former ; crevasses are formed at right angles to 
the direction of tension, and 7¢ zs approximately at right angles to 
the direction of pressure, as tn the case of slate rocks, that the lamination 
of glacier ice is developed. 
Under ordinary circumstances, therefore, the Jamination near the 
1 The possibility of the coexistence of lateral crevasses and compression at the centre 
may, perhaps, be thus rendered manifest :—let a4, cd be two linear elements of a glacier, 
situated near its side SI. 
>_> 
c——d c d' 
S 1 
Suppose, on passing downwards, the line a becomes shortened by longitudinal pressure 
to a’é’, and cd to ¢’d’, which latter has passed a’d’ 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 plain, that though each element has been compressed, the differential motion 
has been such as to dzstend the line 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 will be 
formed. 
2 Philosophical Magazine, 1845, vol. xxvi. 
VOL. II Cc 
