52 GROVE KARL GILBERT— DAVIS 



The phrasing here employed would suggest that after a district had been worn down to a 

 nearly level surface, its even degradation was continued to lower and lower levels. 



THE GILA CONGLOMERATE 



The idea of an uplift of the continental interior after the time of basin-range faulting is met 

 in connection with the dissection of the intermont basin by the Gila headwaters, described in a 

 preceding section. A phrase omitted from the quotation there made is here supplied: 



By the deepening of the draining canon, an excavation probably connected with a broad continental 

 oscillation to which further allusion will be made in the sequel, the water ways have been carried below the 

 floor of the plain (530). 



But unhappily a close scrutiny of the sequel finds no further reference to this important 

 subject than is given in an account of other parts of the Gila River and of the so-called Gila 

 conglomerate, as follows: The upper valleys of the Gila and its branches, declining southwest- 

 ward from the clift southern rim of the plateaus, are occupied by an extensive deposit of con- 

 glomerate, that slopes down to the gravel plains between the basin ranges, but unlike these 

 low-level deposits, which are as a rule still accumulating, the conglomerate is now trenched by 

 valleys to a depth of 1,000 or 1,500 feet. 



It is in its relation to the rivers that it [the conglomerate] is chiefly interesting; in the accumulation, and 

 subsequent excavation of the beds, there is recorded a reversal of conditions, that may have a broad mean- 

 ing. . . . There is no difficulty in comprehending the present action [of the rivers], for it is the usual habit of 

 swift-flowing streams to cut their channels deeper; but to account for the period of accumulation there must be 

 assumed some condition that has ceased to exist. Such a condition might be, either a barrier, somewhere below 

 the region in question, determining the discharge of the water at a higher level than at present, or it might be a 

 general depression of the region, in virtue of which the ocean (now three hundred miles away) became a virtual 

 barrier. With either hypothesis, a change of more than 1,000 feet must be considered (540, 541). 



Here we find not only the intimation of a possible subrecent upheaval of the region, following 

 a time of lower stand or "depression," but again a reference to the ocean as a "virtual barrier" 

 to the erosion of valleys. But the ocean seems to have been in general so remote a contingency 

 that it was seldom mentioned. A possible explanation of the valleys in the conglomerate as the 

 result of a change of relation between load and stream power, due either to variation of climate 

 or to deformation in the headwater region, is not considered. 



SUMMARY 



Although Gilbert adopted a systematic instead of a narrative order in the presentation of 

 his results, he was writing only a report not a textbook, and therefore made no attempt to com- 

 plete the scheme in which all his various items of observation might have been contained. 

 Each item was reasonably explained, and in certain cases the explanation included some account 

 of earlier and of later stages which preceded and followed the observed stage of physiographic 

 evolution, thus introducing the idea of a systematic sequence in the sculpturing of land forms; 

 but the fundamental principle here involved was not emphasized, perhaps because it was not 

 perceived in its entirety. A few steps at a time seems to be the order of progress. The large 

 steps which Gilbert took were in the direction of recognizing that every element of the surface 

 of the lands is the product of some reasonable processes, and of holding the physiographer 

 responsible for the elucidation of those processes. Gilbert's steps, like those taken by PoweU, 

 were manifestly the forerunners of others which later generalized the relations of the various 

 processes as applied to various structures in a more comprehensive evolutionary scheme; and 

 it is for this reason that all later progress in physiography is so deeply indebted to the work 

 of these pioneers in western exploration. 



