LECTURE XI. 317 



glaciers, and above these formations angular blocks were de- 

 posited and pushed forward by the extreme end of the glacier. 

 Escher has shown, that in the whole district of Burgdorf, 

 Wangen, and Langenthal, westward, and eastward over Brugg 

 to Eglisau, there is such an alluvial formation, in which are 

 seen blocks floated by the ice from several basins ; but where 

 they were transported by the solid ice, they are less in num- 

 ber. In the vicinity of these ice-tongues, which persisted 

 during the retreat, have been found lake basins produced in 

 the manner described. The waters may at that time, as Mor- 

 lot assumes, have been one hundi-ed and fifty to one hundred 

 and eighty feet above their present level, and then gradually, 

 when the retreat recommenced, after forming several terraces 

 of about one hundred and then fifty feet above their present 

 height, may have ultimately descended to their present level. 



Finally, there commenced a further retreat after the stop- 

 page. Several terminal moraines show that this retreat was 

 not effected without fresh halting places which correspond 

 with the alluvial terraces on the plains, and each of these 

 stoppages may have continued for a long period, for here also 

 we find moraines of remarkable size, which required a long 

 time for their formation. That during this whole period of 

 retrogression, the formation of glacial clay, of alluvia, of rolled 

 pebbles, and the transportation of angular blocks upon float- 

 ing icebergs, continued uninterruptedly, is undeniable. 



I am fully aware that my theory is in antagonism with that 

 of many geologists, who assume two separate ice periods, be- 

 tween which the older alluvia are said to have been deposited. 

 Morlot, Collomb, and many others, especially English authors, 

 defend this dualism, whilst Desor has always insisted upon 

 the unity of the glacial period. Both parties agree as to the 

 facts, but not as regards their explanation. The old alluvium, 

 no doubt, overlies everywhere the old glacial clay with the 

 polished rolled stones, and equally without doubt, above this 

 old alluvium lie angular blocks intermingled with glacier clay 

 and recent alluvium. The stratification of the terminal moraines 

 in the valleys and lake basins of level Switzerland above and 

 upon the old alluvium is however nowhere demonstrated, and 



