380 Professors Tjmdall and Huxley on the Structure 



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 g h and i k the longitudinal ten- 

 sion is changed to compression ; the central figure is flattened, 

 while the side ones remain stretched. In the corresponding por- 

 tion of the glacier we should expect the central fissures formed 

 between ef and g h 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 n, the widening of the valley, in the case 

 now in our recollection, causes the ridges produced at the pre- 

 vious 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 reasoningf. 



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 inclina- 

 tion to the axis of the glacier continually changes ; but were the 



* 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. 



-d' 



-I 



Suppose, on passing downward, the line ab becomes shortened by longitu- 

 dinal pressure to a'b', and cd to c'd', which latter has passed a'b' 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 distend the line of particles joining a and d, in the ratio -r?,. 



If this ratio be more than that which the extensibility of ice can permit of, 

 a side fissui'e will be formed, 

 t Phil. Mag. 1845, vol. xxvi. 



