ORIGIN OF TALLEY-LAKES. 75 



and hollows formed by those movements which produced the in- 

 versions here alluded to ? 



4. The theory of subsidence has had but scant justice done to it. 

 Alpine geology opens one's eyes to the importance of this in a 

 way that English geology does not; it is not enough therefore to 

 dismiss this subject on the negative grounds that in England we 

 have nothing more than mere pools formed in this way, where the 

 solution and removal of the salt-beds beneath has led to subsidence. 

 Nor do I see any force in the argument against the theory from the 

 mere number of instances. No one would maintain that all the 

 lakes even of the Northern Alps are due to subsidences ; the point 

 is that here we do find conditions favourable to subsidence extending 

 over hundreds of miles in a district abounding in lakes. The 

 materials of which the mountains are composed are for the most 

 part calcareous and dolomitic in character, the strata being often 

 almost pure limestone or dolomite. There is no necessity for 

 explaining here how underground erosion is carried on by water in 

 such strata ; the fact is well known, as in the case I the famous 

 caverns of Castleton and Adelsberg, to mention on" ^wo most con- 

 spicuous instances. Again, in this very region beds of rock-salt 

 and gypsum abound, some of them of great dimensions. The lateral 

 outflow from the Konigsee through a hole in the mountain, the 

 water passing for miles underground and issuing again as a river 

 in the direction of Hallein, serves very well to illustrate the way in 

 which the waters which have done the work of underground erosion 

 can find their way from these upland valleys, in which most of the 

 lakes we are considering occur, down to lower levels. In some in- 

 stances the actual amount of material carried from the interior of 

 the mountains has been found to be astonishingly large. I shall 

 mention here two examples given by Credner * : — (a) At Neusalz- 

 werk, in Westphalia, it has been found that the brine-springs of 

 that district bring to the surface 376 cubic metres of carbonate of 

 lime annually ; (6) In the Visp-Thal, in Canton Wallis, a series of 

 ' Erdfalle ' accompanied by small earthquakes continued for a whole 

 month in the year 1855, due to the undermining of the valley by 

 the removal -of gypsum in solution. The district contains some 20 

 gypsiferous springs, one of which alone brings to the surface over 

 200 cubic metres of gypsum per annum. Such facts enable us to 

 realize more vividly than any thing which English geology presents 

 to us the importance of underground erosion as a geological factor. 



It is the mere enunciation of an axiom to point out that in all 

 such cases the materials cannot at the same time be removed from 

 the interior of the mountains and remain inside them. Time is the 

 most important factor in such cases. The prevalence of salt and of 

 brine-springs in those northern Alpine regions where the lakes are 

 most numerous is recorded in the frequent occurrence of local names 

 indicative of their presence (e. g. Salzburg, Hallein, Hallstadt) ; and 

 an exploration of any of the salt-mines of the district opens one's 

 eyes to the great extent to which underground excavation has been 

 * Elemente der Geologie, pp. 207, 210. 



