1848.] CURIOUS ANALOGIES IN PLANED IRON. 219 



and feasibility of my anticipation. There is no difficulty in de- 

 termining the exact line of pressure, for it is obviously that in 

 which the tool is made to act, or it is mathematically parallel to 

 the flat side of the shaving itself, if we suppose it straightened. 

 (Fig. 7.) * In one of the specimens now before me, the planes 

 of detrusion, or frontal dip, make an angle, as nearly as can be 

 estimated, of seventy degrees, with the base or line of pressure. 

 From i\\Q fibrous appearance of the whole mass, I have little 

 doubt that it is traversed by numberless fissures or flaws parallel 

 to the planes of actual sliding, flaws which might probably be 

 made evident by immersing the whole in dilute acid. 



Time does not allow me to add more. Some may consider 

 these approximations and analogies trifling, but I persuade my- 

 self that you will not do so, being well aware how much has 

 resulted in the progress of science from the patient study of 

 minute facts not obviously related to one another. It is some 

 pleasure to me to persuade myself that my speculations upon 

 the cause of the motion of glaciers have had some slight in- 

 fluence in drawing attention to the loose manner in which 

 bodies have hitherto been classified as solid and fluid, rigid, 

 flexible, or plastic. On the one hand, attention is directed to 

 the way in which stress or strain is exerted upon masses, and 

 modified by their internal constitution in a way which no theory 

 not embracing an expression of that constitution founded on 

 experience can possibly represent. On the other hand, the im- 

 perfect views which practical men have entertained as to the 

 manner in which intense strains affect materials of certain kinds 

 and in certain forms, are apparently about to undergo a con- 

 siderable revolution. I remain, my dear Sir, yours very truly. 



EDINBURGH, 2d December 1848. 



* [The analogy of this figure to fig. 7 of Plate I., representing the effect of 

 vertical detrusion on a lava stream of Mount Etna, is remarkable.] 



