bo 
42 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
is depressed but three hundred feet at the least and fifteen hundred at 
the most. 
Even yet, however, we have not restored the country to as low a level 
as the facts seem to demand. As we have seen in a preceding section 
(p. 235), the valley of the Virgin river southwest of Toquerville shows. 
signs of a renewal of erosion, which was probably associated with the 
disturbances attendant on the recent faulting. The only satisfactory 
explanation of this seems to be that the western block was upheaved in 
sympathy with the eastern block, but toa much less extent. Along the 
Colorado river and Grand wash there are similar indications that the 
western block was uplifted somewhat, although perhaps less than at 
Toquerville. For the sake of stating the problem in a quantitative form, 
let us assume that the farther depression required by the renewed erosion 
of the western block, in order to restore the region to the attitude which 
it held at the end of the inter-fault cycle, amounts to five hundred feet 
along the Colorado river, and one thousand feet near Toquerville. These 
figures are only approximations, and can be doubled or halved without 
affecting the verity of our presentation. At Toquerville, then, according 
to this hypothesis, the northern end of the Shivwits plateau stood at an 
elevation of from fifteen hundred to two thousand feet. To the south- 
ward it descended gently, until at the Colorado river the elevation of 
the Mohave peneplain was only five hundred, or at most one thousand 
feet. The strata, which now dip gently to the north, must then have 
dipped a little to the south, and near the Virgin river there must have 
been a faint anticlinal arch, with an east and west axis. The Uinkaret, 
Kanab, and Kaibab plateaus, to the east, must have stood very low also, 
since there is no evidence of any marked modern break between them 
and the Shivwits plateau, and since the Grand canyon has been cut in 
them to a depth as great as in the Shivwits, during what appears to be 
exactly the same length of time. To the northwest, however, the Colob 
plateau, and probably the rest of the High Plateaus seem, as measured 
by the amount of faulting, to have stood at an elevation of about six 
or seven thousand feet, only three thousand feet lower than at present, 
but it may be that these figures will require modification. The topog- 
raphy of the plateaus as seen in Colob, and in those described by 
Dutton, is more subdued and more mature than would be expected in 
a region standing at so great an elevation, within a few miles of a 
lowland four thousand feet nearer to sea-level. It is possible that a 
further study of this comparatively little known region will show that 
the amount of uplift at the time of the last faulting was rather more 
