EEV. A. TRYING ON THE 



CQ 



ZQ 



(2) Again, it may well have hap- 

 pened that in some of these later 

 movements of the mountains, lateral 

 thrusts may have acted vertically, 

 or nearly so, to older lines of valley. 

 The effect of such forces would be, 

 if the resultant force was not too 

 deeply seated, to partly close in the 

 valleys upon which they were 

 brought to bear, until the sides of 

 the valleys in their narrowest parts 

 came together, leaving the wider 

 portions to form true rock-basins, 

 which, getting filled with water 

 from the surface-drainage, would, of 

 course, form lakes. The direction 

 of the principal lines of flexure of 

 the Jura chain affords a strong pre- 

 sumption in favour of the hypo- 

 thesis that in this way the valley of 

 the Rhone below Geneva may have 

 been partly closed in. This alone 

 would account for so much of that 

 lake as lies in a true rock-basin. 

 Part of its depth, however, it owes 

 to the delta of the Arve, which has 

 been brought down from the Mont- 

 Blanc region*. 



(3) There is a phenomenon f well 

 known to mining-engineers as a 

 "creep." In old coal-workings it 

 is found that the vertical pressure 

 of the superincumbent strata, acting 

 upon the strata immediately below 

 the galleries where the coal-seams 

 have been removed, causes the floor 

 of the mine to rise, so as in time to 

 completely fill up the disused work- 

 ings. Such a resolution of vertical 

 pressure is possible where more mas- 

 sive strata, as often occurs in the 

 lake-regions of the Alps, rest upon 

 strata of a more yielding nature on 

 opposite sides of a gorge-like valley. 

 In such cases the erosion of the 

 valley being continued down to the 

 lower strata, these, being at the 



* Vide ' Les Causes actuelles en Gre'olo- 

 gie,' by Meunier, pp. 203, 204. 



t Vide Lyell, ' Student's Elements,' p. 56. 



