210 GROVE KARL GILBERT— DAVIS [ME1IOIES [vouxxi, 



flowed directly from the mountains across the weak beds of the longitudinal valley and through 

 notches in the inclosing sandstone ridge, have been diverted to one or another of the larger out- 

 flowing consequents by the normal headward growth of their deeper-lying longitudinal branches. 

 But the text avoids the use of the technical terms by which the several kinds of streams and vallevs 

 may be concisely named, and briefly announces that the outflowing streams "enlarged their 

 valleys in the yielding beds of the Fontaine formation and finally drew off the headwaters of 

 the former streams," the lower courses of which are now marked by wind-gap notches in the 

 Dakota ridge and by crystalline gravels from the mountains farther out on the plains. 



This brief statement is followed by another, which makes assertion instead of giving 

 explanation of the general principles here involved, and which is therefore just about as impen- 

 etrable to the general reader as if it were an explanation couched in technical terms. It is as 

 follows : 



The diversion of these [headwater] streams because they were too weak to keep pace with their neighbors 

 in carving channels through the resistant strata is part of a general process of rearrangement by which the 

 minor elements of drainage are turned away from resistant rocks. The arrangement of small waterways is 

 continually adjusted to the arrangement of resistant rocks. 



Not one nongeological reader in a thousand will, on reading this insufficiently explanatory 

 passage, really apprehend the spontaneous origin of longitudinal streams by headward erosion 

 along belts of weak strata. Even the three figures in the folio text which illustrate the pro- 

 gressive revelation and breaching of a resistant anticlinal formation in the northern part of the 

 quadrangle do not suffice to make clear the development of longitudinal streams within the 

 anticline, although several examples of such streams occur there. A fuller and more explicit 

 explanation of the processes here involved would seem to be quite as appropriate, if they are 

 to be explained at all, as the explicit explanation of so simple a matter as a flood plain, above 

 quoted from the report on underground waters ; and a generic name for the kind of valley that 

 is so well represented in the southwestern corner of the Pueblo quadrangle would seem to be 

 physiographically as desirable as a formational name is geologically desirable for the strata 

 which in the Pueblo quadrangle outcrop only in that valley, but which ara nevertheless very 

 properly given a special designation and a special color. 



There was, of course, an earlier period in the evolution of physiography when neither 

 Gilbert nor anyone else in America recognized the existence, much less the origin, of the kind 

 of streams and valleys here considered. In the article on the "Plateau province as a field for 

 geological study," Gilbert gave no hint that Powell's classification of streams as consequent, 

 antecedent, and superimposed was not as complete as Powell himself thought it to be; but 

 there came soon afterwards a second period in physiographic progress when the streams and 

 valleys that follow along the weak formations in and near the Henry Mountains were under- 

 stood to belong to a different class from the other three, and this understanding appears to 

 have been in mind during the study of the Pueblo quadrangle ; yet curiously enough, as far 

 as Gilbert was concerned, a third period of progress was not reached in which the additional 

 class was thought to be important enough to have a name similar in rank to that of the three 

 names proposed by Powell. 



The reason for this failure is difficult to discover; and to those who, on the one hand, 

 regard Gilbert as a master of physiographic investigation and who, on the other hand, find 

 that their own investigations are aided by giving appropriate technical names to pecuhar 

 classes of streams and valleys, the fact that he did not feel the need of giving such names to 

 other than the three Powelhan classes of streams is somewhat embarrassing. His failure to 

 introduce or to use a special name for streams that have grown spontaneously by headward 

 erosion along weak belts of inclined strata can hardly have been due to a disinclination to 

 coin a new term, for he took that liberty when he felt it was advisable to do so; witness the 

 introduction of "laccolite" in the Henry Mountains report; witness also an earlier statement 

 in the account of monoclinal flexures by which the plateau province is traversed: "It is neces- 

 sary to explain one or two terms, which have had to be coined in order to describe the new group 



