92 Proceedings of the Royal Irish Academy. 



the mass. Part of the tendency thus expressed acts upward (as if the 

 mass were a fluid) chiefly along the shortest line to the surface, as 

 shown by thick arrows in the figure ; and this compounded with the 

 general onward movement of the mass, in a direction roughly indicated 

 by the chain-line, must exercise a scooping action upon the ground 

 beneath, which may account for the special deepening of valleys within 

 their openings, as in the cases of the hollows now occupied by Lake 

 Iseo and Lough Derg. 



Dr. Hess, with other writers on Alpine giaciation, maintains that 

 the V-shaped cross-sections of valleys are attributable, not to water 

 erosion, but to the gouging action of successive glaciers, each producing 

 a U-shaped valley. Four such stages of erosion have been noticed by 

 Penck and Briickner,^ and designated by them as follows: — 



1. Giinz-Eiszeit^ because the fluvio-glacial deposits, the older 

 boulder-clays from the glaciers of this period, are especially well 

 developed on the Iller-Lichplatte in the district of Giinz. 



2. Mindel-Eiszeit, because the deposits of the period, corresponding 

 to the later boulder-clay, are spread out chiefly in the province of 

 Mindel. 



3. Hiss-Mszeit, of which the deposits form terraces in the Kiss 

 valley on the north border of the Ehine. 



4. Wiirm-Eiszeit, represented in the later terminal moraines and 

 terraces in the region through which the TFiirm stream flows in the 

 plains of Munich. They are briefly designated by the letters 

 G, M, H, JF; or y, m, r, tv, for the deposits belonging to each 

 system. 



Hess'^ gives the accompanying section (fig. 1)^ to illustrate the 

 formation of valleys according to this view, and writes as follows 

 regarding them : — ^' In the Stubaital, in the Brenner-furche, in the 

 district of the Zillertal Alps, and in the Perwall, everywhere I found 

 the profile of the valleys the same as in the figure (Yenter Tal and 

 Gurglar Tal) ; everywhere there are four trough -forms lying one 

 within another ; and the edges of the troughs, for individual valleys 

 retain courses almost parallel to the valley bottoms." 



Comparing this view, and the section given, with a cross-section 

 (fig. 2)^ of the gorge above Killaloe, there is a strong temptation to 

 regard its features as due chiefly to three stages of giaciation 



^ Die A Ipen im Eiszeitalter, by Dr. A. Penck and Dr. E. Bruckner. Lieferung I., 

 p. 110. 



- Op. cit., p. 364. 3 See Plate VI. 



