82 ON THE OKIGIN OF VALLEY-LAKES. 



pointed out in the ' Geological Magazine ' for 1875. In lake-glaciers 

 motion is chiefly derived from the expansion of ice in the act of 

 freezing. The water produced by the heat developed by friction 

 during the progress of the glacier freezes in the glacier and causes 

 motion (in the direction of the least resistance) towards the outlet 

 of the lake where the glacier is thinnest. 



Mr. Teall said the burden of proof must rest with those who 

 asserted that a glacier could excavate. The differential motion of 

 a glacier was a most important point. If the ice were moving up- 

 hill it would be stopped by friction and gravitation ; hence it was 

 dim cult to understand how a basin could be excavated, and very 

 strong proof was needed. 



Prof. BoiraET criticised Mr. Tylor's section and idea, and stated 

 that, while agreeing with Mr. Irving in his general principles, he 

 differed as to the details. He thought that Mr. Irving had not 

 allowed for the fact that many of the large lakes lay in true basins, 

 so that he could not explain them by moraines or debris. Still he 

 thought that rock-movements accounted for the larger basins. 



Mr. W. Mathews referred to the recession of the Swiss glaciers 

 during the last few years, and asked if on any rock thus exposed a 

 basin had been seen. 



The Axjthoe, in reply to Prof. Blake, referred to the last discussion 

 and his last paper. He entirely dissented from Mr. Tylor's view 

 of the possibility of the ice doing erosive work by freezing at the 

 base of the glacier ; it would have to expand under great pressure, 

 and so would compress and solidify rather than erode the rocky floor. 

 The question of how much of a lake was true rock-basin and how 

 much formed by moraine or detritus must be worked out in each 

 case separately. 



