146 Review of the Controversy Regarding the Motion of Glaciers. 



forward, must be as great, supposing the motion of the ice to be mole- 

 cuhir in the way I have explained, as it would be supposing the ice 

 descended in the manner generally supposed." * But this claim of 

 Mr. Croll on behalf of his theory can not be admitted. As long as 

 the glacier moves as ice, it is perfectly easy to admit that it may hold 

 stones and sand firmly enough to cause them to score the rocks in its 

 channel rather than to slip from its grasp. But when the glacier 

 is supposed to descend molecule by molecule, and not as ice, but 

 as water, it is impossible to believe that such action is any longer 

 possible. The material carried down by a river never polishes or 

 grooves its channel, if consisting of rock, and the Liquid Theory imagines 

 the glacier to flow down as a river flows, with the sole difference, that 

 its particles move singly, slowly, and intermittently, rather than con- 

 currently fast, and continuously. Moreover, this very objection has 

 always proved a formidable obstacle to the acceptance of Forbes' 

 theory. The slight degree of viscosity demanded by its fundamental 

 principle made it difficult to admit the possibility of such results as 

 grooving and striation. It is not, therefore, necessary to dwell upoii 

 the enhancement of the same difficulty that is introduced by the 

 Liquid Theory in question. 



Mr. Croll further adds : 



" We have in this theory a satisfactory explanation of the origin of 

 crevasses in a glacier. Take for example the transverse crevasses 

 formed at a point where an increase in the inclination of the glacier 

 takes place. Suppose a change of inclination from 4° to 8° in the bed 

 of the glacier. The molecules on the slope of 8° will descend more 

 rapidly than those above on the slope of 4°. A state of tension will 

 therefore be induced at this point, where the change of inclination 

 occurs. The ice on the slope of 8° will tend to pull after it the mass 

 of the glacier moving more slowly in the slope above. The pull being 

 continued, the glacier will snap asunder the moment that the cohesion 

 of the ice is overcome. The greater the change of inclination, the 

 more readily will the rupture of the ice take place." 



Mr. Croll has here fallen into the logical fallacy, so common with 

 the advocate of new theories, of specially (}U0ting, in his own support, 

 a phenomenon that is almost equally favorable to all the hypotheses that 

 have been pnt forward to account for the movement in question. 

 Even if the claim be admitted, it would form no argument in defense 

 of the Liquid Theory, because, it may be made with equal show of 

 reason by the followers of Forbes, and with yet more justice by the 



* L. E. and D. Phil. Mag., March, 1869. 



