340 
PEOFESSOES TTXDALL AXD HUXLEY OX 
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 glacier ice is developed. 
Under ordinary chcumstances, therefore, the lamination near the sides of the glacier 
would, in accordance with the theory of compression, be obhque 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 compression, 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 produces a thrust and compression is also instrumental in developing the 
veined structure. In short, so far as om’ 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 distance upon our own observations alone. Before 
he formed any theory of the structure, and in his first letter upon the subject. Professor 
Forbes remarks, that “ the whole phenomenon has a good deal the air of a stmcture 
mdivjcedi perpendicularly to the lines of greatest pressure"' His later testimony is in sub- 
stance the same. In his thirteenth letter, read before the Royal Society of Edinburgh 
on the 2nd of December, 1846, he says that the blue veins are formed where the pressure 
is most interne. In his reference to the development of the laminar stmctiu’e 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 vemed structure is especially 
developed in front, i. e. against the opposing side of the valley, where the pressime is 
greater than laterally.” In fact, the parallelism of the phenomenon to that of slaty 
cleavage struck Professor Forbes himself, as is evident from the use of the teim “now” 
in the following passage : — “ It will be understood that I do not noio suppose that there 
is any parallehsm between the phenomenon of rocky cleavage and the ribboned structure 
of the ice.” This reads like the giving up of a previously held opmion ; the term noio 
being printed in italics by Professor Forbes himself. The adoption of the Uscous theory 
appears to have carried the renunciation of this idea in its train. 
Later still, and from a source wholly independent of the former, we have received 
additional testimony on the point m question. The follo^ving quotation is from a letter, 
dated I6th November, 1856, received by one of us ffom Professor Clausius of Zmich, 
so weU known in this country through his important memohs on the Mechanical Theorj- 
of Heat : — “ I must now,” writes M. Clausius, “ describe to you another singular coin- 
cidence. I had read your paper upon the cleavage of rocks and it occurred to 
* Travels, p. 352. 
