336 



of their mutual thrust is to develope the veined structure along their 

 line of junction. This is illustrated in the case of the Aar and other 

 glaciers ; and experiments are described which show the mechanical 

 condition of such glaciers, and that the veined structure of the ice, 

 which sometimes rises to a case of " true cleavage," occurs at the 

 precise places where the compression theory would lead one to expect 

 it. The lenticular structure before referred to, the obliquity of the 

 veins to the sides of the glacier, the transverse lamination at the 

 centre, the relation of the blue bands to the crevasses, are all in 

 perfect harmony with this theory. Indeed, unless we suppose the 

 ice of glaciers to be perfectly homogeneous, mechanically speaking, 

 we must infer that under pressure some portions will be rendered 

 more compact than others, and the veined structure is the natural 

 consequence*. 



Finally, the well-known phenomenon of " dirt-bands " upon the 

 surfaces of glaciers is considered, and an explanation of those bands, 

 as seen upon the glaciers of Grindelwald and the Rhone, is attempted. 

 On the former glacier the bands were particularly well developed ; 



* Since the presentation of the paper, I have tried to reproduce the veined 

 structure on a small scale by compressing snow. In some cases the section of 

 the mass perpendicular to the surfaces on which the pressure was exerted, ex- 

 hibited in a feeble but distinct manner an appearance the same in kind as that of 

 the veined structure of the glacier ice ; stripes more transparent than the sur- 

 rounding ice were observed at right angles to the direction of pressure. I have 

 also succeeded in impressing upon a perfectly transparent prism of ice a cleavage 

 which certainly surpassed my expectations. The cleavage, as in the case of the 

 glacier and of slate rock, is perpendicular to the direction of pressure. On 

 placing a specimen of the squeezed mass before a highly competent judge, he at 

 first imagined it to be a piece of gypsum. The full details of these experiments 

 shall be communicated in due time to the Royal Society. The case, then, as 

 regards slaty cleavage and the structure of glacier ice, stands thus : The testimony 

 of independent observers proves that they are both laminated at right angles to 

 the direction of pressure ; and the question occurs, Is the pressure sufficient to 

 produce the cleavage ? Experiment replies in the affirmative. I have reduced slate 

 rock to an almost impalpable powder, and reproduced from it the lamination by 

 pressure. In the glacier we find equally the cleaved structure associated with 

 pressure, and the above experiments prove the sufficiency of the pressure to 

 produce the structure. By combining the conditions of nature, we have produced 

 her results ; and it may perhaps be questioned whether a theory is capable of 

 more convincing proof than that thus furnished. J. T. 



