216 ' National Geographic Magazine. 



dependent on the size and slope of the streams and the resistance 

 of the rocks, the streams will be more or less re-arranged, some 

 of the larger ones persisting in their courses and cutting their 

 channels down almost as fast as the mass below them is raised 

 and offered to their action. It is manifest that streams of large 

 volume and considerable slope are the ones most likely to per- 

 severe in this way, while small streams and large ones of mod- 

 erate slope may be turned from their former courses to new 

 courses consequent on the new constructional form of the land. 

 Hence, after a disturbance, we may expect to find the smaller 

 streams of the former cycle pretty completely destroyed, while 

 some of the larger ones may still persist ; these would then be 

 called antecedent streams in accordance with the nomenclature 

 introduced by Powell.* A fuller acquaintance with the develop- 

 ment of our rivers will probably give us examples of river sys- 

 tems of all degrees of extinction or persistence at times of dis- 

 turbance. 



Since Powell introduced the idea of antecedent valleys and 

 ' Tietze, Medlicott and others showed the validity of the explana- 

 tion in other regions than the one for which it was first proposed, 

 it has found much acceptance. Lowl's objection to it does not 

 seem to me to be nearly so well founded as his suggestion of an 

 additional method of river development by means of backward 

 headwater erosion and subsequent capture of other streams, as 

 already described. And yet I cannot help thinking that the ex- 

 planation of transverse valleys as antecedent courses savors of 

 the Gordian method of explaining a difficult matter. The case 

 of the Green river, to which Powell first gave this explanation, 

 seems well supported ; the examples given by Medlicott in the 

 Himalayas are as good : but still it does not seem advisable to 

 explain all transverse streams in this way, merely because they 

 are transverse. Pei'haps one reason why the explanation has 

 become so popular is that it furnishes an escape from the old 

 catastrophic idea that fractures control the location of valleys, and 

 is at the same time fully accordant with the ideas of the unif orm- 

 itarian school that have become current in this half of our cen- 

 tury. But when it is remembered that most of the streams of a 

 region are extinguished at the time of mountain growth, that 

 only a few of the larger ones can survive, and that there are 

 other ways in which transverse streams may originate,! it is evi- 



* Exploration of the Colorada River of the West, 1875, 153, 163-166. 

 fHilber, Pet. Mitth., xxxv, 1889, 13. 



