academy 0F scENcas] POWELL'S SURVEY 107 



under such headings as declivity, transportation, and balanced action. Indeed, even the idea 

 that underlies the term is mentioned only in connection with river mouths, and curiously 

 enough it is usually the mouths of branch rivers in trunk rivers that are referred to, and not 

 the mouths of trunk rivers in the sea. The latter relation is stated once and empirically in 

 connection with the general problem of the upstream increase of river grade. 



If we follow a stream from its mouth upward and pass successively the mouths of its tributaries, we find its 

 volume gradually less and less, and its grade steeper and steeper, until finally at its head we reach the steepest 

 grade of all (116). 



Farther on the problem is more locally treated in connection with the drainage of the 

 Henry Mountains: 



The streams which flow down them are limited in their rate of degradation at both ends. At their sources, 

 erosion is opposed by the hardness of the rocks. ... At their mouths, they discharge into the Colorado and 

 the Dirty Devil, and cannot sink their channels more rapidly than do those rivers (175). 



Gilbert's whole discussion of land sculpture is a "fresh-water" discussion, and Powell's 

 discussion was very much the same. 



The reason for this would appear to be that both Powell and Gilbert lived and worked 

 within a broad continental area, and not along its ocean borders. Although both of them 

 greatly advanced the physiographic problem of land sculpture in their early reports, neither of 

 them then discussed the action of the waves on shore lines. Gilbert later gave truly enough a 

 most illuminating account of lake shores, as will be told below; but he never gave much atten- 

 tion to continental coasts; and Powell's only concern with oceanic shore lines was of a per- 

 functory kind in one of the short-lived "National Geographic Monographs." The progress 

 of physiography in continental America is therefore just the reverse of its progress in insular 

 Great Britain; for there, on a land fragment where, as one may say, it is hardly possible to 

 escape the sound of the surf, the work of rivers was relatively neglected and the sculpture of 

 inland escarpments as well as the excavation of interior lowlands was long attributed to marine 

 action; while here, in a continent so broad that the very existence of an ocean may be forgotten 

 while the problems of the vast interior are considered, the importance of river action was more 

 fully recognized, and naturally enough the action of ocean waves on continental coasts at the 

 same time was relatively overlooked. 



The absence of the important physiographic factor, time, from Gilbert's reports is more 

 perplexing. He must have known perfectly well that the existing conditions of drainage 

 systems as well as the existing forms of the land surface are the product of erosional processes 

 acting upon structural masses through longer or shorter periods of time; yet his account of 

 streams and of land forms is much more concerned with their existing status than with their 

 evolutionary development from an earlier or initial status into the present status. It is only 

 by reading between the lines that the idea of systematic change with the passage time is to be 

 gathered, and even then but incompletely. The passage about stream volumes and grades 

 quoted in the second preceding paragraph concerns only a maturely developed river system; 

 nevertheless the law of increasing steepness upstream is, without qualification, said to apply 

 "to every tributary and even to the slopes over which the freshly fallen rain flows in a sheet 

 before it is gathered into rills. The nearer the watershed or divide the steeper the slope; 

 the farther away the less the slope" (116). Yet this evidently holds good only for ready-made, 

 full-grown drainage systems, neither young nor old. It is true that the need of much erosional 

 work in the production of a systematic increase of river grade from mouth to source is intimated 

 a few lines later, when it is said that such an arrangement "is purely a matter of sculpture, the 

 uplifts from which mountains are carved rarely if ever assuming this form"; but the idea of 

 development here intimated is not fully carried out. Consideration of the time factor is excep- 

 tional all through the chapter on "Land sculpture." Even the possibility that rivers may 

 grow old and that mountains may be worn down is presented only as an unrealizable tendency: 



It is evident that if steep slopes are worn more rapidly than gentle, the tendency is to abolish all differences 

 of slope and produce uniformity. The law of uniform slope thus opposes diversity of topography, and if not 

 complemented by other laws, would reduce all drainage basins to plains. 

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