152 bulletin: museum of comparative zoology. 



posed). In a district where superposition seemed impossible, all streams 

 that did not follow the dip of the strata in consequent fashion were 

 classified as antecedent. Writing of the Colorado basin, Powell said : 

 " All the facts concerning the relation of the water-ways of this region 

 to the mountains, hills, canons, and cliffs, lead to the inevitable conclu- 

 sion that the system of drainage was determined antecedent to the fault- 

 ing, and folding, and erosion, which are observed, and antecedent, also, to 

 the formation of the eruptive beds and cones " (a, p. 198). Even certain 

 minor streams that follow monoclinal valleys along the northern flank 

 of the Uinta mountains were for this reason explained as having been 

 there before the mountains were raised, and the uplift of the mountains 

 was thought to have been too slow to displace them, in spite of their 

 small volume (a, pp. 159-166). To-day there can be little question 

 that these monoclinal streams are not antecedent but subsequent ; that 

 is, they have gained their position by headward erosion along the strike 

 of the weak strata in which their valleys are eroded — as first explained 

 by Jukes, who wrote nearly forty years ago, " the longitudinal valleys 

 are of subsequent origin" (p. 400) — from time to time capturing and 

 diverting the upper courses of such consequent streams as they encoun- 

 tered, and thus bringing about that remarkable adjustment of streams 

 to structures which characterizes in so high a degree all deeply denuded 

 regions of strong deformation. 



Like Powell, Gilbert recognized three classes of streams, but he seems 

 to have felt some doubt as to the generally antecedent origin of the in- 

 consequent streams. He wrote : " A large share of the drainage of the 

 plateaus is not consequent. How much is super-imposed and how much 

 antecedent remains to be determined " (6, p. 102). 



Dutton was as deeply impressed with the antecedent origin of the 

 Colorado system as was Powell. Not only the trunk river, but most 

 of its branches were thus explained. A consequent origin is ascribed to 

 the lateral ravines which descend the structural slopes of the Kaibab 

 arch (c, p. 195), but an antecedent origin is announced for nearly all the 

 tributaries of the Colorado ; for the San Juan, Little Colorado, and Cata- 

 ract on the south (c, p. 219), and for the San Rafael, Curtis (b, p. 63), 

 Fremont (a, p. 282), Paria, and Kanab on the north (c, p. 188), as well 

 as for the streams that are thought to have once occupied the now dry 

 Summit-valley depressions of the Kaibab (c, p. 193), and the House-rock 

 valley between the Kaibab and Paria plateaus (c, p. 188). 



Replacement of Antecedence by Other Explanations. — The nat- 

 ural history of rivers is to-day better understood than when Powell and 



