Dr. C. S. Du Riche Preller — Zonal Lake Basins. 223 



these two glaciations, which filled the valleys to the altitudes given 

 above. 1 



It follows then (1) that the present lakes were formed during, and 

 primarily in consequence of the last general retreat of the glaciers, in 

 other terms towards the end of the Glacial Period ; (2) that the lakes, 

 like the main rivers which traverse them, were, at the time of their 

 formation, at levels about 150 feet higher than at present, and that 

 they gradually dropped to their present levels as the rivers eroded 

 their beds more deeply in the Alpine valleys above and in the sub- 

 Alpine valleys below them; (3) that the present lakes afford no 

 criterion as to the extent or depth of previous — Interglacial or pre- 

 Glacial — lakes which in any case must have been formed at much 

 higher altitudes. 



III. The Lakes and the Molasse Flexure. 



The lakes having been primarily formed as shown in the preceding 

 paragraph, it remains to explain the great depth of their central 

 portions. In this connexion the theory of glacial erosion must be 

 discarded, for a glacier, though it can scour, abrade, level, and polish, 

 cannot perform the mechanical work of excavation. It has volume, 

 but no effective velocity, and even its volume, not moving as a rigid 

 mass, is rendered ineffective by its constant state of internal friction 

 and deformation. 2 The depth of the lakes could be more easily 

 explained by so many rifts at right angles to the trend of the Alps, 

 were it not that the disruptive agency as the primary cause is 

 wanting. 



The only possible and logical explanation of the phenomenon must, 

 therefore, be sought in a slow, simultaneous subsidence of the lake 

 floors by a zonal bending of the Molasse formation in which the lake 

 basins lie. A line drawu through the deepest points of the five lakes 

 will be found to run parallel to the crest, and along the edge of the 

 Alps, within but close to the western limit of the Molasse formation. 

 This cannot be an accidental phenomenon ; it constitutes, in fact, 

 the syncline of the main flexure of the Molasse as shown in the plan 

 (Fig. IX), p. 219. 



Of this flexure the Molasse strata themselves afford conclusive 

 evidence at various points. Henevier recognized its anticline as 

 running from Savoy across the Lake of Geneva and thence north-east as 

 far as the Canton of Appenzell (Lake of Constance), and deduced from 

 it, though without reference to the lakes, " l'affaissement sur la 

 lisiere des Alpes," which corresponds to Heim's ' Einsenkung ' or 



1 Some Swiss lowland Deckenschotter deposits at a somewhat lower level 

 than the typical high-level ones led Miihlberg, Penck, and Bruckner to assume 

 a fourth glaciation (in the Tyrol even five), intermediate between the first and 

 maximum glaciations ; but those lower deposits may be, like that of 

 Teufelskeller, near Baden, west of Zurich, due to local subsidence. 



2 On the other hand, a big glacier is indirectly a potent factor in valley- 

 making and valley-shaping by the disintegrating action of frost and of the 

 great differences between extreme temperatures on the mountain-sides as well 

 as by its great lateral and vertical pressure. In this action lies obviously the 

 via media between the extreme views pro and contra glacial erosion. 



