572 Notices of Memoirs — James CroU, on Glaciers. 



II. — On the Cause of the Motion of Gtlaciers. 

 By James Ckoll, of the Geological Survey of Scotland. 

 [Philosophical Magazine, September, 1870.] 



THE ice of a glacier is now almost universally believed to be, not 

 a soft plastic substance, but a substance hard, brittle, and un- 

 yielding. The power that the glacier has of accommodating itself to 

 the inequalities of its bed, without losing its apparent continuity, is 

 referred to the property of regelation possessed by ice. All this, 

 says Mr. Croll is now plain ; but what impels the glacier forward is 

 still a question under discussion. The answer generally given is 

 that gravitation alone is the force which does this. But as the ice of 

 the glacier descends with a differential motion, we have not only to 

 explain what causes the glacier to slide on its bed, but also what 

 displaces the particles of the ice over and alongside one another. 

 The Eev. Canon Moseley ^ has lately investigated the cause of the 

 descent of glaciers, and he has found that the amount of work per- 

 formed on a glacier during its descent through a given space is 

 enormously greater than the work of the weight of the glacier 

 descending through that space. He has determined that the aggre- 

 gate work of the resistances which oppose themselves to its descent 

 in a given time is about thirty-four times the work of the weight in 

 the same time ; consequently it is physically impossible that the 

 mere weight alone of the glacier can be the cause of its descent. 



He has further shown that the mere weight of the ice is wholly 

 insufficient to overcome the cohesion of the crystalline particles, so 

 as to break their connexion and cause them to be displaced one over 

 the other. And this point Mr. Croll regards as fully established. 



Mr. Croll reviews Canon Moseley's paper, and also the obser- 

 vations of Messrs. Mathews and Ball, published in reply to it.^ 



It is generally supposed that the ice-particles of a glacier are in 

 a hard, solid, and crystalline state, and that, owing to differential 

 motion, the cohesion of the ice-particles is broken, and that these 

 solid particles are forced over and alongside one another. Two 

 particles separate, and the one moves past the other. " But," asks 

 Mr. Croll, " were the two particles, at the moment when separation 

 took place, both in the hard, crystalline, and solid state ? " Canon 

 Moseley does not prove this ; he merely assumes it to be the case, 

 and shows that if the particles of the ice are in this state, weight 

 is insufficient to produce the descent of glaciers, and, therefore, some 

 force in addition to it is required to cause the phenomenon. Mr. 

 Croll argues that Canon Moseley has not proved his point ; he has 

 only shown, that if the glacier shears in the way that it is generally 

 supposed to do, it cannot descend by its weight alone. Canon 

 Moseley regards solar heat as the primary cause of glacier motion. 

 The fact that a glacier moves more rapidly during the day than 



^ Proceedings of the Eoyal Society, January, 1869. See also Geologicai, Maga- 

 zine, May, 1870, p. 229. 



* In reference to these replies, Mr. Croll states his inability to perceive that any- 

 thing which they have advanced materially affects Canon Moseley's general con- 

 clusions as regards the commonly -received theory of glacier motion. 



