Academy of Sciences] POWELL'S SURVEY 117 



often by wide intervals of time, that the great result is accomplished" (210). If Gilbert thought 

 it desirable in 1880 to write in a manner that seems to-day so elementary, it must be that 

 physiographic geology has made great progress between then and now. 



In consequence of long-continued erosion, partly during, partly after, the time of uplift, 

 the ideal flat-topped dome has been greatly denuded and so far unroofed over an excentric area 

 of 60 by 25 mdes in its eastern half as to expose beneath the cover of the bedded rocks, originally 

 nearly hah a mile in thickness, the nearly vertical quartzites and schists of the Archean, trending 

 northward or northwestward and including lenticular masses of intrusive granite. In this 

 unroofed area the resistant quartzites form "abrupt, dike-like ridges . . . bristling suddenly 

 in peaks" (52, 58), the granites form ridges, peaks, and pinnacles; and slates are excavated 

 in valleys, some of which are wide enough to constitute "beautiful expanses of treeless and 

 well grassed parks" (56). It may be noted in passing that these ridges and valleys afford 

 a fine illustration of what has later come to be called topographic discordance with respect 

 to the scarped edges of resistant overlying limestones. A further consequence of the long- 

 continued erosion is that a series of weak "Red beds," 540 feet thick, lying in an inclined 

 position around the margin of the dome, are now excavated in a "Red valley," known to the 

 Indians as the "Race-course," from 1 to 3 miles wide, which runs conformably between the 

 stripped slope of the underlying limestones, 650 feet thick, and an encircling monoclinal ridge 

 of resistant sandstones, 300 feet thick. 



The first of the two chief factors which determine mountain form, namely, the rock struc- 

 ture of the mass, having been sufficiently treated, attention is turned to the second factor, 

 namely, the arrangement of drainage lines. Here Gilbert's treatment of stream development 

 is much the same as it was in the Henry Mountains report. Distinction is made between 

 the creeks which flow radially outward from the hills on the one hand, and, on the other hand, 

 the two chief rivers of the district, both branches of the Cheyenne, which flow eastward across 

 the southern and northern ends of the domed strata. The creeks are first described empirically, 

 with relation to the dip of the domed strata; they are called cataclinal, following Powell's termi- 

 nology. They are then described genetically with relation to their origin, and called conse- 

 quent. Taken together, they are held to "afford a rare example of consequent drainage"; 

 the encircling monoclinal ridge is cut by them 27 times. The two rivers, running somewhat 

 irregularly into and out from the encircling monoclinal ridge at the two ends of the dome, 

 are not classed as antecedent, as they might at first appear to be, but as superimposed, because 

 the White River beds, the uppermost of the plains strata, are found to lie unconformably on 

 the next lower strata, and to contain a bed of quartz pebbles at their base, derived from the 

 Archean rocks of the hills ; hence it is inferred that all the doming and much of the erosion were 

 accomplished before the deposition of the White River beds ; that those beds were spread uncon- 

 formably over the worn-down margin of the domed strata; and that the two rivers took conse- 

 quent courses on the unconformable cover; and later, as their valleys were incised, found them- 

 selves by superimposition more or less astride of the monoclinal sandstone ridge as it came 

 into relief. The creeks of the unroofed Archean area might also have been classed as super- 

 imposed, as they transect the quartzite ridges in sharp-cut canyons with precipitous walls (59). 

 It is interesting to note that, in connection with the unconformable deposition of the White 

 River beds, the phrase, "base level of erosion" appears to have been used by Gilbert for the 

 first time. His chapter closes with a large generalization regarding the work of the consequent 

 streams : 



The canons they have cut, and which appeal to our eyes as marvellous monuments of their industry, are 

 the least of their results. Since their labor began they have demolished and removed one half of the entire 

 mass of the uplift. Modest and feeble as they seem, it is their ambition patiently to toil on until no vestige of 

 the Hills remains (222). 



In two respects, Gilbert's analysis of the drainage of the Black Hills calls for extension. 

 First, as with the little pairs of short subsequent valleys back of the "revet crags" of the 

 Henry Mountains, so with the somewhat better developed pairs of subsequent valleys that, as 

 branches from the radial consequents, are extended along the weak "Red Beds" around the 



