1846.J THREE ORDERS OF DISCONTINUITY IN GLACIERS. 205 



sents, year after year, compact ridges, such as a b, c d, parallel 

 to one another, and separated by a mass of crevasses which it 

 is, generally speaking, impossible to cross ; and any one who 

 attempts to traverse the more crevassed portions of the glacier 

 without attending to this peculiarity, will infallibly lose his way. 

 But, as the drawing explains, the direction of these ridges by no 

 means corresponds with that of the individual crevasses, whose 

 grouping subdivides the glacier as I have now described. The 

 crevasses may be nearly transverse to the glacier, whilst the 

 systems of crevasses form an angle of perhaps 30 with the 

 transverse line. The veined structure again cuts the crevasses 

 at right angles, so that these may be regarded as three orders of 

 discontinuity, or tearing surfaces, which occur in systems, that 

 is, regularly repeated at nearly uniform intervals over great por- 

 tions of the glacier surface. I have this year succeeded, for the 

 first time, in laying down on a map an approximation to the 

 various and complex systems of crevasses which traverse the 

 Mer de Glace, and I have found a repetition of this phenomenon 

 of a series of discontinuous but parallel fissures ranged along a 

 line or axis oblique to their direction, to recur at several points 

 where the strain is very violent. Let it be remarked, too, that 

 where the violence of the pressure opens a system of such 

 fissures to relieve it, the bands, or system of surfaces of mole- 

 cular discontinuity, disappear, or are less well developed. I 

 remain, etc. 



12th December 1846. 



XVII. FOURTEENTH LETTER ON GLACIERS. 



Addressed to Professor JAMESON. 



On the Variation of the Motion at different Seasons. Observations on the Glacier 

 of the Aar, considered and compared with the Author's results. 



My Dear Sir I am led to request you to reprint in the 

 Edinburgh Philosophical Journal, part of the 9th section of my 

 paper on the Viscous Theory of Glacier Motion from the Philo- 



