ACADEMY F SCIENCE] POWELL'S SURVEY 103 



done this. When the bottom of the canon was a thousand feet higher the creek failed, at a place where the dip of 

 the strata was comparatively small, to shift its channel as it deepened it, and began to cut its way into the mas- 

 sive [underlying] sandstone [on the west]. Having once entered the hard rock it could not retreat but sank 

 deeper and deeper, carving a narrow gorge through which it still runs making a [3-mile] detour from the main 

 valley (137, 138). 



The main monoclinal valley has been weathered and washed out along the streamless inter- 

 vening space to almost the same depth as farther north and south where the creek runs. Such 

 is the origin of the little Horseshoe Canyon, figured in an upstream or northward view on page 138 

 of the report. 



Now if the drainage of this flexure were stated in accordance with the present terminology 

 of stream and valley lines, the headwater branches of the Dirty Devil where they cross the 

 flexure would, like the radial streams of Mount Ellsworth, be called consequent; and Hoxie 

 Creek, manifestly developed by the headward erosion of a branch of the Colorado along the belt 

 of weak shales in the great flexure, would be called subsequent, except that its detour through 

 Horseshoe Canyon might be described as having been locally superposed by planation during 

 a time of wandering in a late stage of a former cycle of erosion — probably the cycle that preceded 

 the current cycle of canyon incision. But Gilbert did not use these terms. The terminology 

 that he employed is best understood by reviewing the earlier stages in his treatment of stream 

 and valley lines. 



His first stage is found in the Wheeler reports, where certain valleys that follow the strike 

 of weak strata were in some cases at least given the empirical or structural name of monoclinal 

 valleys, as has been already told. A second stage was reached after he joined the Powell survey, 

 when, curiously enough, he used "consequent" in a peculiar sense in a notebook, under date of 

 August 18, 1875; after giving a brief account of Horseshoe Canyon and another but smaller 

 detour of Hoxie Creek he wrote : 



It is a strange watercourse, consequent (following a mono) for most of its course, but inconsequent in two 

 places. 



That is, he regarded the main course of the creek as " consequent " not upon the initial slope 

 of the great flexure but upon the monoclinal belt of weak shale laid bare between the overlying 

 and underlying sandstones; and the two detours were regarded as "inconsequent" because 

 they departed from the monocline. Such a terminology was perfectly consistent within itself; 

 but it was peculiar in leaving its first term without explanation, although an explanation for its 

 second term — superposition by planation — was suggested in the final report. 



A third stage of treatment in terminology exhibits progession in certain respects, and 

 retrogression in another. It is found in the Henry Mountains report, where Powell's terms 

 are adopted as already noted. Here "consequent" is employed, as quotations given above 

 clearly show, for streams that still follow courses originally determined by the initial slope of 

 a deformed land surface, like that of a laccolithic arch or a monoclinal flexure, and this was a 

 forward step ; but singularly enough no term is introduced to express the other relation between 

 stream and structure previously attached to "consequent" in its notebook meaning, and this 

 was a backward step. Thus while Gilbert did not commit the positive error made by Powell 

 earlier and by Dutton later of naming a monoclinal stream like Hoxie Creek "antecedent," 

 he unfortunately overlooked the descriptive use of "subsequent" in an account of the erosional 

 origin of monoclinal streams by Jukes 15 years before, and he committed the negative error 

 of not inventing any other genetic name for streams and valleys of this class. The reason for 

 this omission appears to be that subsequent streams like Hoxie Creek had not run across his 

 path frequently enough to hold his attention upon them. It is true that the subsequent .mono- 

 clinal ridges and divides, the converse of subsequent monoclinal valleys and streams, which 

 encircle a number of the Henry Mountains domes, were seen, described, explained, and named, 

 but the little subsequent valleys between the ridges and the domes were not especially con- 

 sidered, as will appear from the following passages in the report. 



