1857.] * Observations on Glaciers. ' 325 



box, with a sluice front, containing fine mud, formed of a mixture of 

 pipeclay and water. The mud was permitted to flow slowly through 

 the trough. Two highly coloured straight lines ab^ cd, were 

 stamped on the mud transverse to the length of the trough, and 

 two others m n, op, were drawn parallel to its length. Now, if 

 filaments move past each other towards the centre, such a motion 

 must be observed by its effect upon the longitudinal lines m n, op. 

 On permitting the mud to flow, the lines lower down presented the 

 appearance shown in the figure ; m n and o p moved parallel to the 

 sides, and at the same distance from them from top to bottom ; 

 there was no evidence whatever of the supposed drag : the differ- 

 ential motion which existed was parallel to the length of the 

 trough, as is proved by the distortion of the lines a b, cd, while 

 every point in each of the longitudinal lines moved exactly as the 

 theoretical deduction from Fig 1 would lead us to expect. If 

 then the cleaved structure were due to differential motion, we 

 ought to expect it parallel to the sides, instead of oblique to them, 

 as it actually exists in a glacier of the form typified by our wooden 

 trough. 



Finally, with regard to the transverse lamination of the centre 

 of the glacier, the theory now under consideration assumes, that in 

 a mass supposed to be viscous, with an enormous thrust behind, and 

 an enormous resistance in front, fissures varying from a fraction of 

 an inch to several inches in width are formed at right angles to the 

 direction in which the thrust is exerted. Surely, so far from pro- 

 ducing such fissures, the direct effect of such a force would be to 

 close them up if they existed. 



The speaker next attempted to apply the theory of slaty clea- 

 vage, already referred to, to the laminated structure of the ice. 

 The lamination, like that of slate rock, is always approximately at 

 right angles to the direction of maximum pressure ; this fact is 

 established by the testimony of independent observers, and was 

 first, it was believed, recognised by Professor Forbes himself. 

 Local circumstances, which give rise to a violent thrust, produce 

 at the same time a highly developed lamination. When two con- 

 fluent glaciers unite to form a single trunk, as in the case of the 

 Finsteraar and Lauteraar glaciers, the effect of their mutual 

 thrust is to develop the veined structure in a striking degree along 

 their line of junction. The mechanical condition of such a 

 glacier was illustrated by experiments with mud, flowing through 

 two branches, which afterwards were united in a single trunk. 

 Small red circles were stamped all over the surface of the mud in 

 the two branches; and as these descended they were squeezed 

 along the centre of the trunk into ellipses so elongated that the 

 conjugate axis in many cases disappeared wholly, and the figure 

 became a straight line. In nature it is exactly at the places where 

 this squeezing takes place, that the cleavage of the ice is most highly 

 developed; a fine example of this is the structure under the central 



