184G.] DISTRIBUTION OF CREVASSES. 145 



the patient and disinterested labour of that competent sur- 

 veyor. 



But if this be true in a merely superficial plan, how much 

 more true would it be if we could pare off the upper stratum 

 of the glacier, and view a horizontal section of it at a depth of 

 a hundred feet ! The depth of the crevasses has, I am per- 

 suaded, been as much exaggerated as the thickness of the ice 

 of the glacier has been underrated. In how few cases (where 

 a glacier does not descend tumultuously) can we let a plumb- 

 line down even fifty feet without grazing the sides ! and to 

 what an insignificant fissure has the gaping crevasse dwindled 

 even at that small fraction of the glacier's thickness! Supposing 

 the crevasse to become uniformly narrower, how soon would it 

 be extinct ! 



Again, the crevasses which traverse the surface of the 

 glacier, have almost always a determinate direction or direc- 

 tions, of which the simplest type seems to be that of perpen- 

 dicularity to the veined structure,* which, generally speaking, 



* [Travels in the Alps, p. 171. As the differential velocities considered in 

 vertical plane, to which the frontal dip of the veined structure (see above, pp. 19, 59) 

 is principally due, are for the most part caused by intense resistance in the direction 

 of motion, the pressures accompanying or producing these differential velocities do 

 not tend directly to produce crevasses unless the glacier be free to spread itself 

 laterally. In this latter case the crevasses open parallel to the movement of the 

 ice or to the length of the glacier, and we have then the radiant system of cre- 

 vasses already several times referred to, as in the glacier of the Rhone, etc. (pp. 7, 

 20). The powerful lateral compression exerted by the confining walls of the Mer 

 de Glace, the glacier of the Aar, and other canal-shaped glaciers, prevent this mode 

 of fissure. Under these circumstances, as I have pointed out in a paper in the 

 Philosophical Magazine for May 1845 (which being chiefly controversial, is not re- 

 printed at length here), we must consider the perpendicularity of the crevasses 

 to the veined structure to have reference to the differential motions which produce 

 the veined structure estimated in a horizontal plane, or disembarrassed from the 

 effect of hydrostatic compression producing the frontal dip.f I have in the same 

 place illustrated the correctness of this view by a reference to the plastic models 

 described in 1 of the present paper, where, practically, the frontal resistance 

 does not interfere with the result, and where, therefore, the lines of fissure or cre- 

 vasses due to tension, are throughout perpendicular to the lines of differential 

 motion or the veined structure due to horizontal forces only. The direction of 

 these last, as well as of the crevasses perpendicular to them, are sufficiently indi- 

 cated in figs. 2 and 6 of Plate I., and also in the woodcut on page 79 of the 

 present volume. Nov. 1858.] 



f Phil. Mag., May 1845, p. 408. 



