Mr. J. Ball on the Formation of Alpine Lakes. 493 



glacial mud or fine debris. If in this imaginary case the slope 

 of the supposed valley were reduced successively to 4°, 3°, and 

 2°, we should find that the period necessary for the operation of 

 removing the vast mass of underlying matter must be prodi- 

 giously increased, as the increasing resistance offered by friction 

 would diminish to a small fraction the onward motion of the 

 ice upon its bed. If the slope were but 1°, we should hesitate to 

 believe in the possibility of such a removal within any calculable 

 lapse of time. Underlying rocks may be scored and glaciated 

 by the under surface of a glacier, though it should advance but 

 a few inches in the year ; but the destruction and removal of 

 vast masses of mineral matter require that the motion by which 

 they are effected should be of appreciable amount. 



It is needless to say that the above argument applies with 

 still greater force to a glacier supposed to lie on a dead level of 

 vast extent. My conviction is that under such circumstances 

 the resistance, offered by the bed would be far greater than that 

 arising from the internal cohesion of the ice. In such a case 

 the upper portions of the glacier would flow over the inferior 

 portions, and the bottom would remain fixed on its bed. Let it 

 not be forgotten that in the case of a monstrous glacier lying in 

 a level channel 47 miles long, gravity would no longer have the 

 least action in urgiDg forwards the lower strata of the ice. The 

 only force that we can imagine to act in impelling these forwards, 

 would be that arising from the onward motion of the glacier in 

 the upper alpine valley before it reached the level channel. But 

 before we can admit that pressure can be transmitted through a 

 vast mass of glacier-ice sufficient to overcome an enormous re- 

 sistance, as though the whole were a rigid mass, we must burn 

 the observations of Forbes, Agassiz, Tyndall, and other glacier- 

 inquirers, and forget all the results they have recorded. Pro- 

 fessor Tyndall found that in one portion of the Mer de Glace a 

 slight increase of resistance to the advance of the glacier caused 

 the longitudinal compression of a section of the glacier 1000 

 yards in length at the rate of 8 inches daily ; and we are asked 

 to believe that an enormously greater resistance, spread over a 

 space 47 miles in length, would not prevent the transmission of 

 force through the lower strata of the ice sufficient to overcome 

 the obstacles to its onward movement. Of course the argument 

 here offered applies a multo fortiori if we suppose the bed of the 

 glacier to be concave, instead of being merely a level surface ; but 

 there is one further consideration which seems to be, if possible, 

 still more conclusive. 



Let us suppose that in the case here chosen for consideration 

 the glacier had by some process, to me inconceivable, cleared 

 out one cubic mile of the diluvium from the lake-basin, and 



