1846.] THRUSTS AND TENSIONSFORMATION OF CREVASSES. 151 



departed so far in my Seventh Letter on Glaciers, published 

 subsequently, as to deduce in an approximate manner, from 

 elementary mechanical laws, the directions of the surfaces of 

 tearing within such a mass as I had described, upon the simple 

 supposition that the hydrostatic pressure acting uniformly, the 

 tendency of motion of any particle will be in the direction of 

 least resistance when all the resistances are taken into account, 

 and that the surfaces of rupture will divide particles whose 

 motions are dissimilar, but will not divide particles whose 

 motions are alike. I repeat that I had no reason to repent of 

 my abstinence from theorizing, when I found that a far better 

 mathematician than myself, taking up the inquiry where I had 

 left it, and after applying himself for a long time to the exclusive 

 mechanical considerations which the viscous theory had suggested, 

 Jeft the subject, as I conceive, little more advanced than he had 

 found it, and fell into some mistakes and inconsistencies, almost 

 inseparable from this way of treating a problem which extensive 

 observation and patient thought can alone disentangle. 



Formation of Crevasses. It has been seen in the third sec- 

 tion of this paper, that De Saussure, and almost all his succes- 

 sors, have regarded the crevasses as accidents of glacier motion, 

 and not essential to it ; and in this view I of course concur. 

 Nevertheless, the study of crevasses is one of considerable, 

 though secondary interest, and is very far indeed from being 

 completed. It requires, among other things, a very sedulous 

 attention to the state of the glacier at various seasons, and even 

 whilst covered with snow ; and it requires further a two-fold 

 classification of crevasses, into those which may be considered 

 as proper to the mass of the glacier, and those which merely 

 seam its surface. 



I will first speak of the last point. 



Though the formation of a crevasse betokens a local dis- 

 tending force, such a force cannot with any certainty be referred 

 to the whole depth of the glacier below the point where the 

 chasm opens. On the contrary, there is a fully greater proba- 

 bility that under that very spot the ice is compressed. If one 



