224 GLACIAL GEOLOGY. 



are composed mostly of materials which have beeu detached from the 

 underlying rocks by the erosive action of the glaciers themselves. 

 Their observations show that the regions studied by them in great 

 detail were almost completely buried under ice, so that the accumula- 

 tion of superficial moraines was, for the most part, impossible; and 

 they advance a number of facts which i^rove positively that the ground 

 moraines were formed and accumulated under the ice. These geolo- 

 gists do not deny that some of the material may occasionally have come 

 from above, nor do they doubt that preexisting masses of rock rubbish 

 and alluvial accumulations may have been incorporated with the ground 

 moraines ; but the enormous extent of the latter and the direction of 

 transport and distribution of the erratics which they contain can not 

 be thus accounted for, while all the facts are readily explained by the 

 action of the ice itself, which used its subglacial debris as tools with 

 which to carry on the work of erosion. 



Professor Heim and others have frequently asserted that glaciers 

 have little or no eroding power, since at the lower ends of existing 

 glaciers we find no evidence of such erosion being in operation. But 

 the chief work of a glacier cannot be carried on at its lower end, where 

 motion is reduced to a minimum, and where the ice is perforated by 

 sub-glacial tunnels and arches, underneath which no glacial erosion 

 can possibly take place; and yet it is upon observations made in just 

 such places that the principal arguments against the erosive action of 

 glaciers have been based. - - - If we wish to learn what glacier-ice 

 can accomplish, we must study in detail some wide region from which 

 the ice has completely disappeared. Following this plan, Dr. Blaas has 

 been led by his observations on the glacial formation of the Inn Valley 

 to recant his former views, and to become a formidable advocate of the 

 very theory which he formerly opposed. To his work and the memoirs 

 by Penck, Briickner, and Bohm, already cited, and especially to the 

 admirable chapter on glacier erosion by the last-named author, I would 

 refer those who may be anxious to know the last word on this much- 

 debated question. 



The evidence of inter-glacial conditions within the Alpine lands con- 

 tinues to increase. These are represented by alluvial deposits of silt, 

 sand, gravel, conglomerate, breccia, and lignites. Penck, B<3hm, and 

 Briickner find evidence of two interglacial epochs, and maintain that 

 there have been three distinct and separate epochs of glaciation in the 

 Alps. Ko mere temporary retreat and re-advance of the glaciers, ac- 

 cording to them, will account for the phenomena presented by the in. 

 ter-glacinl deposits and associated morainic accumulations. During 

 interglacial times the glaciers disaiipeared from the lower valleys of the 

 Alps; the climate was temperate and probably the snow-fields and 

 glaciers approximated in extent to those of the present day. All the 

 evidence conspires to show that an interglacial epoch was of i)rolouged 

 duration. Dr. Briickner has observed that the moraines of the last 



