166 PROF. E. J. GARWOOD ON THE [May I906, 



have us regard the majority of the large Alpine lakes as due to the 

 direct erosive action of ice, has gradually given rise to a detailed 

 consideration of the genesis of each individual lake. 



At the present day, several eminent authorities still attribute the 

 origin of some of the larger Swiss and Italian lakes to ice-erosion. 

 Thus Prof. Bruckner, in his recent work, 1 considers that the Lake of 

 Zurich was created by glacial overdeepening, and that it represents 

 the end of an old valley which was overdeepened by theLinth Glacier; 

 he also attributes the formation of the Lakes of Zug and Lucerne to 

 the same agency. 2 Prof. Davis again, in his paper on i Glacial 

 Erosion in the Valley of the Ticino,'" 3 attributes the rock-basin of the 

 Lago Maggiore to direct ice-erosion. It is evident, therefore, that 

 this theory is in no wise extinct. On the other hand, many Swiss 

 and Austrian geologists, notably Prof. Heim and Dr. Forel, 4 

 attribute all these lakes to post-Miocene earth-movements in one 

 form or another. These lakes, which are of considerable size, 

 all occur in the outer zone of the Alps, or the ' Kalk-Alpen ' of 

 Continental writers, and are often spoken of as ' Eandseen ' or 

 marginal lakes. It is not, however, with these that I am at present 

 concerned. 



The lakes, the origin of which I intend to discuss in the present 

 communication, are those to which in this country the name 'tarn' 

 is frequently applied ; they occur invariably in mountain-districts, 

 and in the Alps are confined to the ' Hochgebirge.' They may be 

 defined as lakes draining into the principal Alpine valleys, whereas 

 the Eandseen receive the drainage of these valleys. 



These tarns, although they frequently occur as ' corrie '-lakes, 

 include also others which, strictly speaking, do not come under 

 this category. Of these latter, some are true rock-basins ; while 

 others owe their origin to the damming of valley-drainage by loose 

 material. 



]S"ot long ago, Dr. Marr, in writing of the ' Tarns of Lake- 

 land,' pointed out that many, if not all, of them were due to 

 accumulation, and expressed a doubt as to whether any lakes 

 of this class would eventually prove to be true rock-basius. What- 

 ever may be the case in Cumberland, there can be no doubt of the 

 occurrence of true rock-bound tarns among the ' Hochgebirgseen ' 

 of the Alps. 



In the pages of the 'Geological Magazine' for 1898," Prof. Bonney 

 instanced a group of lakes in the neighbourhood of Airolo, which he 

 unhesitatingly described as true rock-basins, and referred provi- 

 sionally to excavation by ice ; though, in the case of Lake Bitom, at 



1 A. Penck & E. Bruckner, ' Die Alpen im Eiszeitalter ' Leipzig, Lieferung 5 

 (1903) p. 525. 



2 Op. cit. pp. 537-88. 



3 ' Appalachian vol. ix (1900) pp 151-52. 



4 A. Heim, 'Die Entstehung der Alpinen Eandseen' Vierteljahrsschrift 

 der INa turf or sen. Gesellsch. in Zurich, vol. xxxix (1894) p. 66 ; and F. A. Forel, 

 Bull. Soc. Yaud. Sci. Nat. ser. 3, vol. xxvi (1890-91) Proc.-verb. pp. xii, xvi 

 & ' Le Leman ' vol. i (1892) pp. 211 et seqq. 



5 P. 15 [not p. 45, as stated in the index of that volume]. 



