12 /. W. Judd — On Yolcanos. 



ferent manner from that maintained by the advocates of the doctrine 

 of ice-erosion, or even from that suggested by M. de Mortillet. 



In considering this question, it must always be borne in mind how 

 rapidly the effects of subterranean forces on the drainage of a district 

 are masked and concealed by the action of denuding causes. As we 

 sail over the great Alpine lakes, we are constantly impressed by the 

 fact that, even in the case of those of most profound depth, every 

 tiny streamlet that descends from the surrounding mountains is 

 pushing a delta boldly into its waters, while the larger streams have 

 often produced alluvial flats of enormous extent, that have evidently 

 been reclaimed from the area of the lake. To the eye of a geologist, 

 indeed, almost every lake may be said to be visibly filling up; and the 

 whole Alpine System is encircled by innumerable extinct laTces, belong- 

 ing to various geological periods. The effects produced by local sub- 

 terranean movements in the line of a river- valley — whether in creat- 

 ing an increased fall, and thus originating rapids and waterfalls, or in 

 arresting the drainage at certain points, and thus forming lakes — 

 must be regarded as bringing about a condition of unstable equilihrium 

 in the valley ; while the erosive and transporting action of the stream 

 is continually tending to remove the temporary derangements in the 

 system of drainage by the cutting back and levelling of the preci- 

 pices over which rapids and cascades descend, and by filling up the 

 beds of lakes or cutting through the dams that retain them. In 

 those valleys, indeed, wherein the action of denuding forces more 

 than counterbalances that of subterranean movement, the formation 

 of rapids and of cascades on the one hand, and of lakes on th« other, 

 will be prevented. 



In this admirably adjusted system of mutually antagonistic agencies 

 — those namely of surface erosion and subterranean movement — the 

 occurrence of a period characterized by glacial conditions will pro- 

 duce an interruption, which must be attended with very marked 

 though temporary effects. The depressions which under ordinary 

 circumstances would form the beds of lakes, and then rapidly be 

 filled with sediments, would probably be occupied by inert masses of 

 ice, over which the glaciers would flow in just the same manner as 

 the waters of some of the existing Alpine rivers pass over the surface of 

 the lakes that lie in their course, without producing any appreciable 

 effect on the great mass of cold water that occupies their profounder 

 abysses. This temporary arrest of the compensating effects of river 

 action in a valley — while the antagonistic agent, subterranean move- 

 ment, remained unaffected — would of course result in the formation 

 and preservation of a greater number of lake-basins on the one hand, 

 and of abrupt slopes on the ather, than could be originated under 

 ordinary conditions. 



These considerations, taken in connexion with the frequency of 

 the arrest of drainage by moraine matter, enable us to understand 

 that frequency of lakes and tarns in glaciated districts, to which such 

 importance has been attached by the advocates of the theory of ice- 

 erosion.^ But they also afford an equally simple explanation of 



^ Mr. Scrope has called my attention to the interesting circumstance that in both 



