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, cafions, 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 ” (4, 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 (4, 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 
