494 Mr. J. Ball on the Formation of Alpine Lakes, 



that it were still enabled to exert some grinding action upon the 

 remaining mass, which, at the lowest estimate, must have mea- 

 sured 14 cubic miles. By what imaginable means could this 

 enormous mass of matter have been removed from the hollow 

 trough in which, by hypothesis, it was contained ? In the sup^ 

 posed case of a glacier lying in a valley over a stratum of inco- 

 herent materials, the streams formed under the ice would carry 

 off the triturated materials as fast they were formed ; but no 

 such agency can be admitted here. The stream that would issue 

 from such a basin would be merely the overflow of the water 

 rising above the margin. My imagination fails to discover any 

 process by which this could carry off the mineral contents of the 

 basin, and continue to do so till this was drained to a depth ex- 

 ceeding 2600 feet. 



M. de Mortillet seems to suppose that I am not familiar with 

 instances where the glaciated condition of the rock shows that 

 the under surface of a glacier has ascended over a projecting 

 surface. Such cases must be familiar to all alpine travellers, 

 and may sometimes be traced in connexion with existing glaciers; 

 but to infer from these that the entire mass of a glacier could 

 have been urged for miles uphill, with its under surface sliding 

 over the bed, is much as if I should conclude that because a 

 waggon continues to roll down an incline, although a wheel may 

 now and then pass over a stone, the same waggon might of its 

 own accord travel up the incline, where all four wheels must 

 ascend at the same time. When the force of gravity acting on 

 an entire section of the glacier tends to urge it in the same 

 direction as a force acting a tergo, it appears that the tena- 

 city of the ice is sufficient to carry particular portions over 

 convex projections in the bed. But we know from direct ob- 

 servation that the tenacity and rigidity of glacier-ice are confined 

 within narrow limits; and if the force of gravity acting on a 

 considerable section of the glacier were ever to act in opposition 

 to the force a tergo, the ice acted on by these opposing forces 

 would necessarily move upwards and outwards in the direction 

 of least resistance. 



I am well aware that to many geologists arguments derived 

 from mere physical reasoning are not fully satisfactory, and that 

 to such minds the observation of facts seeming to have a direct 

 bearing on the point in dispute will be more conclusive. On 

 this account I desire to mention some observations made during 

 the last summer on a specific point, as to which M. de Mortillet 

 conceived that he had given a satisfactory answer to an objection 

 urged by me. 



In the article already referred to, I made the sufficiently 

 obvious remark that if the lake-basins had once been filled by 



