90 VISCOUS THEORY OF GLACIEK MOTION. [1845. 



violently checked by the resistance of the projecting mass, has 

 been torn up into longitudinal shreds, which from imperfect 

 fluidity have not reunited, but have left open cavities of the 

 form represented in the figure, which exhibit with, remarkable 

 fidelity the forms of the fissures with which glaciers are some- 

 times traversed, when they are subjected to sudden transitions 

 in their states of motion (as in the Glacier des Bossons at Cha- 

 mouni, and which coincide in direction with the veined struc- 

 ture, and pass into it by imperceptible gradations. 



5. What I have called the frontal dip of the veined struc- 

 ture in glaciers,* I have explained by the accumulation of a 

 sluggish mass of considerable extent upon a floor or bed offering 

 the resistance of intense friction ; in consequence of which the 

 mass of ice, urged downwards and forwards by its intense 

 weight, being resisted by the friction of that which immediately 

 precedes it, must yield in the direction of least resistance, or 

 squeeze itself in a slanting direction forwards and upwards, and 

 thus sliding over the resisting mass immediately in front, will 

 produce surfaces of discontinuity or differential velocity in that 

 direction. Such a result I inferred from general principles, 

 without reference to any particular example, and the explana- 

 tion of the superficial convexity of the lower part of many 

 glaciers was evidently satisfactorily explained by it. 



The convex swelling form of a viscous stream will depend 

 principally upon the relative measure of two quantities, the 

 stiffness or viscosity of the fluid, and the inclination of the 

 surface ; although it will also depend on the part of the 

 stream, whether near the origin or the termination, which we 

 consider. 



I have found this variation from concave to convex, depend- 

 ing upon circumstances, alike in glaciers and lava streams. 

 Some very highly inclined small glaciers existing at considerable 

 heights, and therefore very hard and consistent, are, nevertheless, 

 deeply concave from end to end, the slope compensating for the 



* See my Travels in the Alps, 1st edition, pp. 167, 376, and letter to Dr. Whe- 

 well in Jameson's Journal. Oct. 1844, [page 59 of this volume.] 



