196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the explanations now offered and select the survivors from among them. 

 The work of new observers is especially needed in the examination of 

 certain structures, as in the neighborhood of Toquerville, and between 

 Pipe spring and the Toroweap ; and in the discussion of dates of certain 

 past phenomena, such as flexing, faulting, and uplifting; and in the dis- 

 covery of the origin of the drainage system and the character of the 

 climate of past periods. Whether the conclusions here announced shall 

 then stand or fall in whole or in part, it would be a great reward to the 

 writer if they might afford later students of the region even a small 

 share of the aid that he has derived from the earlier work of Newberry 

 and of Powell, of Gilbert and of Dutton. 



The results reached in the foregoing pages may be summarized as 

 follows. There is some probability that the San Rafael swell, like the 

 Waterpocket flexure, is of pre-Tertiary origin. The other deformations 

 of the region, both flexures and faults, are almost exclusively of much 

 earlier date than the canyon cycle, and they may have been formed 

 relatively early in the erosional history of the district. The total denu- 

 dation of the region thus far accomplished may be considered in two 

 parts, of which the first — the great denudation — was far advanced 

 before the general uplift by which the second — the erosion of the 

 canyon and the stripping of weak strata from the plateaus — was intro- 

 duced. But the great denudation was complicated by repeated move- 

 ments, after each of which the processes of erosion may have reached an 

 advanced stage before the occurrence of the next series of disturbances. 

 It is only by an analysis of these repeated movements and revived 

 erosions that the origin of the drainage system can be determined. 

 As far as this analysis can be attempted at present for the Grand 

 canyon district, the side streams seem to be of various origins, except 

 that none of them appear to be antecedent. The Colorado itself may 

 be in part antecedent to some of the many dislocations that the district 

 has suffered, but it seems to be for the most part consequent on the 

 displacements caused by faulting in the later part of the great denuda- 

 tion, and on the form that the surface had assumed at that time. 

 The floor of the Toroweap valley is higher than the neighboring valley 

 floors, because it is sheeted with heavy lava flows which have effectively 

 withstood the intermittent erosive efforts of wet-weather floods. The 

 past climate of the region cannot be safely determined ; a change from 

 a humid to an arid climate at the close of the Miocene does not appear 

 to be demanded by the facts that have been appealed to in its support. 



