I INACCESSIBLE VALLEYS 17 



Brighton and Beachy Head, instead of following the low 

 ground and reaching the sea between Eastbourne and 

 St. Leonards ; while the Avon, which flows through the 

 gorge of St. Vincent's rocks at Clifton, might apparently 

 have found a much easier way to the sea by a more 

 northerly or a more southerly course. Mr. Jukes ex- 

 plains all these cases on the principle that the courses of 

 almost all the rivers of a country were determined by the 

 contour of the land when it first rose above the sea, the 

 surface water seeking always the easiest course along the 

 hollows and gentle slopes, without any regard to the 

 nature of the rocks beneath. When once these streams 

 had formed definite channels, it was almost imj^ossible to 

 alter them (except when diverted by lava streams or 

 glaciers), because movements of elevation are so slow that 

 the rivers can cut their way down as fast as the land 

 rises up. Thus, the American geologists have proved that 

 the Uintah Mountains were upheaved across the valley of 

 the Green River after the course of that river was 

 established, and that, as fast as they rose, the river cut 

 through them, and now flows in a tremendous gorge or 

 canon. Another illustration of the permanence of river 

 channels is afforded by the Moselle, which, although it 

 flows at the bottom of a deep, narrow valley sunk in a 

 nearly level plateau, winds about in great curves and deep 

 horseshoe bends exactly like a stream flowing over a flat 

 alluvial plain. No explanation of this can be given 

 except that the river began its existence on a nearly level 

 surface, and after it had established its course in the 

 characteristic winding fashion of such streams, it has, in 

 the course of long ages, cut its way deep down through 

 the rock, and thus formed its present valley. 



Now, every considerable area of continental land is 

 made up of rocks and deposits of very unequal hardness 

 and resisting power, from clays and sands to the various 

 kinds of crystalline rock. Some of these can be dissolved 

 and carried away by running water much more quickly 

 than others; while rain, frost, and wind, also act upon 

 their exposed edges very unequally. Hence arise the 

 peculiar forms assumed by hills of different composition, 



VOL. I. C 



