VIII ' RIVERS AND LAKES 319 



back, they would be dammed up, and lakes 

 would result. 



Moreover, if the formation of a mountain 

 region be due to subsidence, and consequent 

 erumpling, as indicated on p. 217, so that the 

 strata which originally occupied the area A B 

 C D are compressed into A' B' C D', it is 

 evident, as already mentioned, that while the 

 line of least resistance, and, consequently, the 

 principal folds might be in the direction A' B', 

 there must also be a tendency to the forma- 

 tion of similar folds at right angles, or in the 

 direction A' C' Thus, in the case of Switzer- 

 land, while the main folds run south-west by 

 north-east there would also be others at right 

 angles, though the amount of folding might be 

 much greater in the one direction than in the 

 other. To this cause the bosses, for instance 



— at Martigny, the Furca, and the Ober Alp, 



— which intersect the great longitudinal val- 

 ley of Switzerland, are perhaps due. 



The great American lakes also are probably 

 due to differences of elevation. Eound Lake 

 Ontario, for instance, there is a raised beach 

 which at the western end of the lake is 363 



