GROWTH OF GEOLOGY. 263 



a relatively recent Great Ice Age, probably inter- 

 rupted by mild periods, and that there had been 

 glacial action even in geological antiquity, were 

 gradually accepted as well-established conclusions. 

 There sprang up, however, a memorable controversy 

 as to the part glaciers had played in gouging out 

 Alpine lakes, valleys, and fiords. To some it 

 seemed that this erosive action -which Gabriel de 

 Mortillet (1858) was one of the first to expound 

 was a certainty; to others, such as Heim, glaciers 

 were regarded rather as conservative than as de- 

 structive agents. Modern opinion has inclined 

 strongly, though not unanimously, in favour of the 

 theory that glacial erosion has been a very important 

 sculpturing factor. 



Professor James Geikie's Great Ice r Age may be 

 mentioned as a crowning work of the nineteenth 

 century study of glaciation, as a modern critical de- 

 velopment of the work of Agassiz and Charpentier, 

 and as a fascinating contribution towards the solu- 

 tion of earth-sculpture. Geikie argues in favour 

 of the conclusion that there must have been six 

 post-Tertiary glacial periods with intervening 

 times of mildness, but as to this, and as to the extent 

 to which glacial periods may be recognised in ear- 

 lier ages, there remains much difference of opin- 

 ion. 



The " drift " which spreads over Northern Eu- 

 rope, with its boulder-clays, erratic blocks, moraines, 

 and the like, admits of only one interpretation, 

 that it is the residue of glacial action. The polished 

 and striated or often much broken rocky floor on 

 which the deposits rest ; the rounded and abraded 

 roches moutonnees; the arctic marine shells found 

 in the drift of Britain, etc., up to heights of 



B 



