164 ORIGIN OF VA1LEYS. 



tion ; when arrested by any obstacle, or in a basin, it would of 

 preference cut through deposits of sand and gravel. We see the 

 contrary of this natural action : valleys do not generally follow the 

 real slope of the soil ; it is not by the lowest part of basins that 

 waters are generally turned, nor through moveable formations that 

 they make a passage. Rivers, in place of having excavated their 

 beds, as was thought, are simply directed by the canals they found 

 already made. Now it is not difficult to go back to the origin of 

 these canals ; they are evidently the result of upheavals, which 

 have embossed or ridged the soil, until then horizontal. It is clear 

 the inflexible beds must have been broken, and consequently a 

 number of cracks were formed, as in the transverse section (Jig. 

 272). The cracks became valleys, placed in different relations to 



Fig. 272. Production of valleys by dislocation. 



each other according to circumstances of upheaval: parallel if the 

 action, taking place in a certain direction, extended a sufficient 

 length ; divergent, if the action occurred at one point, as in certain 

 massive mountains ; often perpendicular to the direction of uplifted 

 chains, as the secondary cracks manifested during earthquakes 

 (Jig. 255), which occurs especially when the internal action forces 

 crystalline matter through the principal crack. It may be easily 

 conceived that crevices would remain more open in solid matters 

 than in arena'ceous deposits., the falling of which would tend to 

 fill the vacancy ; and this is the reason why rivers seem to shun 

 moveable formations, which they could easily excavate if they had 

 not found a bed ready prepared in another direction. 



19. It must not be concluded, however, that water has no agency 

 in the configuration of valleys. On the contrary, we must believe 

 that when a country has been suddenly rent, causing the accumu- 

 lated waters to flow all at once, that torrents of frightful po\ver 

 were produced, tearing away and removing all parts fractured by 

 upheaval, and they thus modified the passages offered to them. 

 It is probable, also, that certain valleys, which pass through a 

 moveable formation, little disposed to fracture, have been produced 

 exclusively by water. Valleys referable to this origin are very 

 different in character from the first : they follow the natural line 

 of slope ; they change their course on meeting masses which offer 

 resistance, and turn round them to remain constantly in the movea- 

 ble deposits. Such are the valleys which cut through the great 

 deposits of rolled flints found at the foot of the oriental Alps. 



19. How are valleys of erosion produced ? 



