226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
sandstone, probably Colob, that appears under the lava sheet, shows a 
very steep dip towards the west, although a little farther west in a. 
second exposure this has greatly decreased. This sudden change of dip 
suggests that under the lava between the two exposures there may be 
another old fault in addition to the one seen at the sulphur springs. Of 
this, however, there is no proof; and the steepness of the dip alone may 
be sufficient to bring the Colob sandstone to its present low level on the 
downthrown side. The displacement as measured along the fault in the 
canyon is relatively small, bringing Aubrey against Moencopie. The 
entire displacement, however, including that due to flexing, amounts to 
thirty-five hundred feet, the vertical measure of the strata between 
the Aubrey and Colob formations. 
From Virgin canyon northward as far as Toquer hill, the old fault 
seems to follow the Hurricane escarpment ; for just south of Toquer hill, 
a few hundred yards west of the Aubrey wall, there are several exposures 
of red Kanab and white Colob sandstone that dip steeply west and are in 
places capped by gravel and lava. They are like the exposures of down- 
thrown strata in the Virgin canyon,.and seem to occupy a corresponding 
position on the same side of the old fault. 
From Toquer hill northward to Kanarra, the old fault is covered, for 
a distance of twenty-five miles, under a belt of basalt and alluvium that 
stretches along the base of Bellevue ridge and of the escarpment of the 
Colob plateau. That the fault is merely concealed and does not die out 
is indicated by the fact that the displacement between the strata west 
of the lava belt and those of Bellevue ridge east of it is much more than 
that of the lava which is displaced by the new fault. Moreover at 
Kanarra an old fault is clearly distinguishable in the strata at the base 
of the Colob plateau. It crosses the prominent escarpment of the new 
fault at a very slight angle. To the northeast it cuts the overturned 
strata of the closest of the folds that have been described above, and 
seems to have displaced them many thousands of feet. At the line of 
the new fault it is cut off sharply as if by a knife. It seems highly 
probable that from Toquerville to Kanarra the old fault increases rather 
than decreases. 
The Inter-Fault Cycle of Erosion. 
After the completion of the first faulting the Plateau province entered 
upon a long cycle of quiet erosion which was brought to an end by a 
second period of faulting. During this inter-fault cycle the work of 
