138 SCIENCE IN SHORT CHAPTERS. 



differ as to the mode and place of its deposition." He 

 argues that it was formed under glaciers of the thickness he 

 describes, while their whole weight rested upon it. 



This appears to me to be physically impossible. If such 

 glaciers are capable of eroding solid rocks, the slimy mud of 

 their own deposits could not possibly have resisted them. 

 The only case where this might have happened is where a 

 mountain-wall has blocked the further downward progress 

 of a glacier, or in pockets, or steep hollows which a glacier 

 might have bridged over aud filled up; but such pockets 

 are by no means the characteristic localities of till, though 

 the till of Switzerland may possibly show examples of the 

 first case. The great depth of the inland lakes of Norway, 

 their bottoms being usually far below that of the present 

 sea-bottom, is in direct contradiction of this. * They should, 

 before all places, be filled with till, if the tillVere a ground 

 moraine formed on land; but all we know of them confirms 

 the belief that the glaciers deepened them by erosion instead 

 of shallowing them by deposition. 



Mr. Geikie's able defence of Ramsay's theory of lake- 

 basin erosion is curiously inconsistent with his arguments in 

 favor of the ground moraine. 



I fully concur with Mr. Geikie's arguments against the 

 iceberg theory of the formation of the till. This, I think, 

 he has completely refuted. 



Before concluding I must say a few words on those curi- 

 ous lenticular beds of sand and gravel in the till which ap- 

 pear so very puzzling. A simple explanation is suggested 

 in connection with the above-sketched view of the forma- 

 tion of the till. All glaciers, whether in arctic or temperate 

 climates, are washed by streamlets during summer, and 

 these commonly terminate in the form of a stream or cas- 



* The largest of the Norwegian lakes, the Mjosen, is 1550 feet deep, 

 and its surface 385 feet above the sea-level. Its bottom is about 1000 

 feet lower than the sea outside, or 500 to 800 feet below the bottom 

 of the Christiana Fjord. The fjords, generally speaking, are very 

 much shallower near their mouths than further inland, as though 

 their depth had been determined by the thickness of the glaciers 

 flowing down them, and the consequent limits of flotation and de- 

 position. 



