Dr. C. S. Du Riclte Preller — Zonal Lake Basins. 221 



including the Lake of Zug as a former integral part, of six distinct 

 basins. Of these, three are longitudinal, and the other three 

 transversal basins, the whole forming an irregular cross whose point 

 of intersection marks the point of greatest depth. The original 

 course of the Reuss was through the Schwyz valley along the 

 northern base of Rigi and through the Lake of Zug, which course, 

 in the lapse of time, it left to follow the southern base of Rigi 

 and erode the defile immediately below Lucerne. Here it struck, 

 and was deflected at right angles by, the River Emme, of whose 

 junction with the Reuss, much larger and at a higher level than at 

 present, the small Rothsee is a remnant. The effect of the Reuss 

 changing its course was the lowering of the level of the Lake of Zug 

 and the severing of its connexion with the Lake of Kusnacht. The 

 plan shows the former extent of the intercommunicating basins in 

 the trough between Rigi and Pilatus, in which the Burgenstock 

 and Horw ridges appeared as islands. The shrinkage of the lakes 

 amounts to more than one-half of their former aggregate length. 



But the shrinkage of the lakes is not confined to their upper and 

 lower ends, for, as shown in the sections, the lakes are gradually being 

 filled up within their present areas, not only by the material 

 transported and deposited in them by the main rivers, but largely by 

 the delta deposits of the direct affluents of the lakes, 1 and in addition, 

 by the lake deposits per se, the lake chalk or ' alluvion impalpable ', 

 which covers the lake floors in some cases to an unknown depth. 



From these considerations it follows: (1) That, under the 

 climatic conditions of the present time, the lakes are in course of 

 being gradually filled up, and that this process would be much more 

 rapid were it not for the great depth of their central portions and for 

 their artificial preservation by canalization of the inflow and outflow. 

 (2) That any lakes existing before the last glaciation were probably 

 filled up and reduced to river valleys much more rapidly and 

 completely than the present ones, 3 a fortiori because they were 

 probably large, shallow, and often intercommunicating expanses of 

 water rather than circumscribed lakes, in which the Alpine rivers, in 

 their then erratic courses and subdivided channels, deposited material 

 much more copiously than the rivers of the present time. 



II. Lakes and Glaciations. 



It has been affirmed by no mean authority 3 that the glaciers passed 

 over, and deposited their moraines at the lower ends of the lakes and 

 thus preserved them \_sic~\. Even if the present lakes had existed 

 before, and had placidly endured throughout the Ice Age, the 

 proposition of their being bridged by the glaciers, as it were, in 



1 The most formidable of these deltas is probably that of the Drance at 

 Thonon (Savoy), -which projects already 2 kilometres into the Lake of Geneva. 



2 Of this, striking evidence is afforded by the interglacial gravel beds near 

 Zurich, the material of which, being strictly fluviatile, can only have been 

 transported across the filled-up lake by a river. 



a Heim, Gletscherhunde, 1885, p. 542; also Favre, R6cJierch.es Geol., 1867, 

 p. 210. 



