54 GROVE KARL GILBERT— DAVIS [MBU0,M [vto£sxi; 



truth of his hasin-range theory resulted from the inclusion in it of a new and essentially physio- 

 graphic principle of his own discovery. Those who had advanced the opposing theory were 

 uninformed as to this principle and could therefore make small use of it; and disagreement 

 necessarily followed. The leading characteristic of Gilbert's principle in its application to the 

 basin-range problem was that it led him to take fuller and more reasonable account of surface 

 features than was the habit of the time. He was not satisfied merely to ascribe the existing 

 forms of the ranges to unspecified erosion, as was then the fashion; he sought to discover the 

 conditions under which the observed forms could be systematically accounted for by the pro- 

 gressive action of erosion upon definite structural masses; and as these sought-for conditions 

 included not only the changing attitude of the structural masses, but also the passage of time, 

 they naturally constituted, when discovered, essential contributions to the history of the moun- 

 tains concerned. 



It is chiefly by reason of his new principle that Gilbert's work on the basin ranges occupies 

 an important place in the history of earth science, and especially of its physiographic chapter. 

 There was an earlier time when geographers took little or no account of the origin of mountains 

 and other features that they described, and when it was the habit of geologists also to give little 

 heed to surface forms and to occupy themselves chiefly with underground structures. Next 

 followed a middle time when, geographers still remaining for the most part indifferent to the 

 rational aspects of land sculpture, geologists at least recognized that surface forms are the result 

 of erosion, although they seldom specified its amount or traced its action. This was truly an 

 advance from a stage of greater indifference, but the advance did not suffice to provide 

 thoroughgoing explanations for land forms of different kinds; and as to the progressive evolu- 

 tion of land forms through a systematic sequence of changes by the work of erosional processes 

 upon one structural mass or another, no such philosophical scheme was broached by the 

 geologists of that physiographically medieval era; or as Gilbert would have said in character- 

 istic western phrase: "You can't prove it by them." Then came a later time when physiogra- 

 phers — that is, geologists with a geographical leaning, or geographers with a geological training — 

 undertook, while accepting crustal structures without particular inquiry into the manner of their 

 origin or deformation, to give special attention to the origin and description of all surface forms 

 as resulting from various amounts of erosion by various kinds of erosional processes upon 

 various sorts of structures; and eventually the erosional processes and the eroded forms 

 were not only systematized as to their kinds and their limiting baselevels, but also as to 

 their variations with the passage of time. Thus the evolutionary physiography of 

 land forms has come to replace the old-fashioned empirical physical geography of the lands. 

 Gilbert's contributions to this advance were invaluable; they "started a ferment in men's 

 minds"; but they were not all made during his membership on the Wheeler survey. 



THREEFOLD TREATMENT OF SURFACE FORMS 



It is true that certain essential steps of the advance were not clearly apprehended, much 

 less explicitly formulated, by Gilbert in his first studies in the West, even though their partial 

 recognition furnished him with a new means of interpreting mountain history. Nevertheless, 

 the evolution of rational physiography, as above outlined, was greatly promoted by the beginning 

 that he then made; for as already intimated, his results imply that he more or less consciously 

 possessed the knowledge and made the application of an important physiographic principle 

 which, even to-day, is by no means so widely understood or so generally employed as it 

 should be; namely, that every structural mass of the earth's crust must have a surface form 

 which is subject to change by deformation and erosion; and consequently, that a physiographer 

 is responsible for the explanatory description of the existing surface form of every crustal mass in 

 terms of its latest deformation and its present stage of erosion, just as a geologist is responsible for 

 the description of its underground structure in terms of its original accumulation and its later 

 deformation; the geologist and the physiographer each being free, of course, to make whatever 

 use of the facts and inferences in the other's special province that may be helpful in reaching 

 his own object. 



