394 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 9, 



admitted." I can only reply by advancing a theory — a theory I call 

 it, because at present I cannot give a rigid demonstration, indeed, 

 have little hope of ever being able so to do. But, as it seems to me, it 

 can be proved by reductio ad absurdum. If a thing can only be black 

 or white, and you prove it cannot be the one, it must be the other. 

 If there are only n possible ways of accounting for a phenomenon, 

 and n — 1 are demonstrated impossible, it follows that the remaining 

 one is correct. This, then, is the method which I chiefly employ 

 here. At the beginning we threw aside, as erroneous, all theories 

 professing to account for lake-basins except two — that which re- 

 garded them as results of glacier erosion, and that which considered 

 them produced by inequalities of motion in a vertical direction in 

 the beds of preexisting valleys, which had been produced in the 

 ordinary way by rain and rivers*. We have now rejected the 

 former ; the latter only remains. I am, however, bound to show that 

 this is not inconsistent with ascertained facts. This I shall endea- 

 vour to do as the conclusion. 



Let us consider first the northernmost group of lakes. It is 

 evident that a very old system of valleys exists here — as may be seen 

 from the tongue3 of Marine ScJiiehten (Middle Tertiary) extending 

 southwards in the lower ground into the Wiener Sandstein (Cre- 

 taceous), showing that certainly the Traunsee, and very probably 

 the Atter See, lie in valleys of early Tertiary age. Yet, though 

 under these circumstances we should expect that the water would 

 pass away northward, we have the stream from the Zeller See 

 descending southward to the Mondsee, and then turning back to 

 pass away through the Atter See ; similarly the "Waller See (a small 

 lake further to the west) discharges its waters into the Salza. These 

 phenomena would be very well explained by a disturbance of the 

 surface to the extent of a few hundred feet vertical, so as to form 

 one or two rolls parallel to the main chain. The Wolfganger See 

 lies in an old, and possibly to some extent pre-Cretaceous, synclinal 

 with a general E. and W. direction. The western end of the lake, 

 which runs about W.N.W. and E.S.E., is in Dachsteinkalk ; but at 

 the eastern end we find the Hauptdolomit appearing, while a little 

 beyond, as well as at Ischl, the Hallstadter Schichten make their 

 appearance, which can hardly be explained without some disturb- 

 ance. Again, at the northern end of the Hallstadter See, and at the 

 gorge of Lauffen, its natural termination, we have Hauptdolomit, 

 the lake itself lying mainly in Dachsteinkalk. There are indeed no 

 marked signs of disturbance visible in the cliffs, though there are 

 many small flexures in the generally horizontal beds ; yet as the 

 general succession of the beds northwards in this region is an ascend- 

 ing one, there appears to be some disturbance here. 



But the strongest case of all is at the Konigsee. Ascending the 

 southern scarp of the massif out of which it is excavated, we pass 



* I wish it to be understood that I do not exclude folds or cracks in 

 the earth's surface, or marine currents as possible originators of valleys ; but 

 I mean that their present contours are wholly due to meteoric action, except so far 

 as they have been affected by slight upheaval or subsidence. 



