and Motion of Glacieis. 381 



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 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 it is approximately 

 ■ at right angles to the direction of pressure, as in the case of slate 

 rocks, that the lamination of glaciei' ice is developed. 



Under ordinary circumstances, therefore, the lamination near 

 the sides of the glacier would, in accordance with the theory of 

 compression, be oblique to the sides, which it actually is. It 

 would be transverse to the crevasses wherever they occur, which 

 it actually is. If the bed of a glacier at any place be so inclined 

 as to cause its central portions to be longitudinally compressed, 

 the lamination, if due to compi'ession, ought to be carried across 

 the glacier at such a place, being transverse to the axis of the 

 glacier at its centre, which is actually the case. This relation of 

 the planes of lamination to the direction of pressure is constant 

 under a great variety of conditions. A local obstacle which pro- 

 duces a thrust and compression is also instrumental in deve- 

 loping the veined structure. In short, so far as our observations 

 reach, wherever the necessary pressure comes into play, the 

 veined structure is developed; being always approximately at 

 right angles to the direction in which the pressure is exerted. 



But we will not rely in the present instance upon our own 

 observations alone. Before he formed any theory of the struc- 

 ture, and in his first letter upon the subject, Professor Forbes 

 remarks, that " the whole phsenomenon has a good deal the air 

 of a structure induced perpendicularly to the lines of greatest 

 pressure." His later testimony is in substance the same. In 

 his thirteenth letter, read before the Royal Society of Edinburgh 

 on the 3nd of December, 1846, he says that the blue veins are 

 formed where the pressure is most intense. In his reference to 

 the development of the laminar structure on the glacier of the 

 Brenva, the pressure is described as being " violent," the effect 

 being such as to produce " a true cleavage when the ice is broken 

 •with a hammer or cut with an axe." So also with regard to the 

 glacier of Allalein*, he says "the veined structure is especially 

 developed in front, i. e. against the opposing side of the valley, 

 where the pressure is greater than laterally." In fact, the paral- 

 lelism of the phfcuomeuon to that of slaty cleavage struck Prof. 

 Forbes himself, as is evident from the use of the term " now " in 

 * Tnivels, p. '.\b2. 



