~ 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. 
— inn, ial 
