18 ON THE STRUCTURE AND MOTION OF GLACIERS 
sides of the glacier would, in accordance with the theory of com- 
pression, 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 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 
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 exertéd. 
But we will not rely in the present instance 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 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 2nd of December, 1846, he says 
that the blue veins are formed where the pressure ts most intense. In 
his reference to the development of the laminar structure on the 
glacier of the Brenva, the pressure is described as being “ vzolent,” 
the effect being such as to produce “a true cleavage when the ice ts 
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, z.e. against the opposing side of the valley, where 
the pressure 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 term “now” in the following 
passage :—“It will be understood that I do not zow suppose that 
there is any parallelism between the phenomenon of rocky cleavage 
and the ribboned structure of the ice.” This reads like the giving up 
of a previously held opinion; the term zew being printed in italics by 
Professor Forbes himself. The adoption of the viscous 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 in question. The 
following quotation is from a letter, dated 16th November, 1856, 
received by one of us from Professor Clausius of Zurich, so well 
1 Travels, p. 352. 
