INTRODUCTION. 



more or less developed into a surface feature in its 

 whole length, and not only in part thereof? This, 

 however, is easily answered, as water flowing down a 

 slope denudes the weakest place, which must neces- 

 sarily be a joint-line or some such crack, and 

 eventually forms a channel along it. To this channel 

 it is confined till it meets another channel, cut by a 

 superior stream, into which it naturally flows, and 

 loses all power of continuing its original course. 

 This, however, is not always the case, as a stream 

 coming down a joint-line belonging to one system 



Bird's-eye Plan of the Victoria Falls and River, after a Sketch 



in the Dublin Exhibition, 1872. 

 a Victoria Fall ; a b c d e, river ; //////, joint lines. 



may meet a joint-line belonging to another which is 

 better developed and more easily denuded, in con- 

 sequence of which it will leave its original course and 

 take to another. This is well illustrated at the great 

 Victoria Falls, Africa, where the water flows over into 



