1846.] CHARACTER OF CREVASSES CHANGES WITH THE SEASON. 153 



remain a fissure in the ice for ever after, why is the horizontal 

 projection or ground-plan of the crevasses of a canal-shaped 

 glacier convex towards the origin of the glacier, and not protu- 

 berant in the direction of its motion, as the ascertained greater 

 velocity of the centre would assign ? Why are the crevasses 

 for the most part vertical and not inclined forwards, or at least 

 not notably so, on the same account ? Why, if the glacier be 

 urged downwards by a longitudinal force distending it, do not 

 the crevasses continually widen in proportion as they are further 

 from the origin ? These questions seem incapable of a sound 

 answer except by supposing that the crevasses are, at least in a 

 great degree, the fresh production of every spring, and arise 

 from the sudden start which the glacier makes when that 

 extremity which descends into the valley begins to experience 

 the thawing effects of returning summer. I should not wish to 

 speak positively upon what involves a difficult if not impossible 

 observation, the state of the glacier with respect to crevasses 

 whilst still under the winter's covering of snow. But the fact 

 of the transverse direction of the crevasses, or even their con- 

 vexity towards the origin, from year to year, seems to admit of 

 no other explanation. But besides this, I can affirm, from a 

 careful observation of the crevasses of the Mer de Glace from 

 June to September in one year, that the changes which they 

 underwent were such as preclude the possibility of a cre- 

 vasse of autumn being merely preserved by the snow of winter, 

 and re-appearing afresh in spring as it had done the previous 

 one. The thing is impossible, because the character of the cre- 

 vasse is essentially altered. In order that an autumnal crevasse 

 may become a spring crevasse, it must be sealed up, annihilated, 

 and opened again. A glance at the three sections in Plate II. 

 fig. 4, will illustrate this. No. 1 shows crevasses freshly opened 

 soon after the snow has quitted the surface of the ice the 

 edges are sharp, the sides vertical, the openings so small that 

 they may be easily stepped across, and in other instances they 

 are not wider than may admit the blade of a knife. No. 2 shows 

 the crevasse opened to its widest extent by the acceleration of 



