152 VISCOUS THEORY OF GLACIER MOTION. [1846. 



cause of a crevasse be, as is universally acknowledged, a pro- 

 tuberance or inequality in the bed over which the ice is impelled, 

 for the same reason that a beam, [supported at one end and] 

 broken by means of weights, is in a state of longitudinal com- 

 pression below, where its surface is concave, and of distension 

 above, where its surface is convex, the cracks in the glacier may 

 be due solely to this last and partial cause. Superficial crevasses 

 may consequently be occasioned where there is no general dis- 

 tension of the mass, either (1.) By the shoving of the semi-rigid 

 glacier as a whole, over a convex declivity; or (2.) From an internal 

 turgescence arising from hydrostatic pressure, resisted by the 

 intense friction of the anterior or more advanced parts of the 

 glacier, which, causing the line of least resistance to be upwards 

 and forwards, forces the pasty mass to tumefy or increase in thick- 

 ness, exactly as it has been seen in 2 [of this paper, page 91], 

 that sluggish lava streams do in a similar case. But if the tume- 

 faction be pushed beyond the limits of plasticity of the superior 

 and more distended -portions, they must burst and assume the 

 crevassed forms actually observed in the plastic models described 

 in p. 78. Hence the existence of crevasses not only does not 

 always result from a state of general distension in the glacier, 

 but may arise from the precisely contrary condition of great 

 internal compression. This argument is well illustrated by the 

 recent observations of M. Agassiz's co-operators on the glacier 

 of the Aar, whose observations I have elsewhere shown* to be 

 incompatible with any other view than that of intense longitu- 

 dinal compression in the mass generally, and yet the surface 

 abounds in crevasses of the usual form and dimensiens.f 



The manner of formation of crevasses generally, including 

 such as may betoken a real distending force acting on any part 

 of a glacier throughout its thickness, is not only a most curious 

 question in itself, but suggests others which a correct theory of 

 glacier motion can alone answer. If a crevasse once formed 



* Ninth Letter on Glaciers. [See p. 70 of this Volume.] 



f [This seems too strongly stated. There are few glaciers less crevassed than 

 that of the Lower Aar, and especially towards the middle of its breadth. Nov. 1858.] 



