over the point under consideration. Until such con. lit 

 of affairs shall have arrived the vertical and lateral n 

 of transporting strength (and hence of corrasive strength) 

 are not expended. 2 Thus to deepen the base of either a 

 lake or fiord basin or of an over deepened valley generally, 

 at any point, one may consider that all the mass of the 

 glacial cross-section is gathered at its centre of gravity. 

 Then stated in other terms one may say that, in order to 

 be able to corrade either the base of the cross-section (or 

 of the basin as a whole) the glacier must be able to trans- 

 port the whole mass of the cross-section, or of the basin, 

 if such mass be gathered at the centre of gravity of cross- 

 section or basin, respectively. Thus at the height of the 

 most recent Ice Flood, the glacier may be considered to 

 have had the whole mass of a lake or fiord cross-section 

 gathered at its centre of gravity, and to have carried it 

 along thence without any difficulty. Now for the sake of 

 simplicity, consider the cross-section of a lake basin below 

 base level to be semi-circular, then its centre of gravity is 

 three-eighths of the depth of the basin from the surface. 

 (Big. 1.) 



A A C C 



Therefore a force which 

 f the mass of ice of 

 iass is gathered at O 



ABC,' 

 titre of gravity, cam 



