228 PROFESSOR JAMES GEIKIE, LL.D., D.C.L., F.R.S., ETC., ON 



and certainly would be insufficient to cause the glaciers of 

 Algau to descend to the foot of the mountains, as we know 

 they did — a distance of at least 24 miles. The Imberg 

 lignites, therefore, are evidence of a climate not less 

 temperate than the present. More than this, there is clear 

 proof that the interglacial stage was long continued, for 

 during that epoch the lller had time to effect very consider- 

 able erosion. The succession of changes shown by the 

 sections near Sonthofen are as follows : — 



1. The lller Valley is filled with glacier-ice which flows 

 out upon the low grounds at the base of the Alps. 



2. The glacier retreats and great sheets of shingle and 

 gravel are spread over the valley. 



3. Coniferous forests now grow over the surface of the 

 gravels ; and as the lignite formed of their remains attains a 

 thickness of 10 feet in all, it obviously points to the lapse of 

 some considerable time. 



4. Eventually the forests decay, and their debris is buried 

 under new accumulations of shingle and gravel. 



5. The lller cuts its way down through all the deposits to 

 depths of 680 to 720 feet. 



6. A glacier again descends and fills the valley, but does 

 not flow so far as that of the earlier glacial stage. 



In this section, as in those at Diirnten and Utznach, we 

 have conclusive evidence of two glacial epochs, sharply 

 marked off the one from the other. Nor does that evidence 

 stand alone, for at various points between Lake Geneva and 

 the lower valley of the Inn similar interglacial deposits 

 occur. Sometimes these appear at the foot of the mountains, 

 as at Morschweil on Lake Constance, sometimes just within 

 the mountain area, as at Imberg, sometimes far in the heart 

 of the Alpine lands, as at Innsbruck. Professor Penck has 

 further shown, and his observations have been confirmed by 

 Bruckner, Blaas, and Bohm, that massive sheets of fluviatile 

 gravel are frequently met with throughout the valleys of 

 the Alps, occupying interglacial positions. These gravels are 

 exactly comparable to the interglacial gravels of the Sont- 

 hofen sections. And it has been demonstrated that they 

 occur on two horizons, separated the one from the other by 

 characteristic groundmoraine or boulder-clay. The lower 

 gravels rest on groundmoraine, and the upper gravels are 

 overlaid by sheets of the same kind of glacial detritus. In 

 short, three separate and distinct groundmoraines are recog- 

 nised. The gravels, one cannot doubt, are simply the 



