HUNTINGTON AND GOLDTHWAIT: THE HURRICANE FAULT. 235 
relation of the two faults at Kanarra indicates that they are of different 
age. The topography seems to show that one is ancient, the other 
modern, and that between the two there was a long period of erosion. 
(5) Evipence or ProLoncep Erosion iN THE DowntTHRowNn Biock 
West oF THE Hurricane Fauut. Inthe preceding paragraphs most of | 
the evidence on which we have based our conclusions as to the amount 
of erosion during the inter-fault cycle has been drawn from the upheaved 
block east of the fault. If our theory is correct, the downthrown, or St. 
George, block on the other side ought to preserve much more perfectly 
the topography of the close of the inter-fault cycle, provided that this 
western block, too, has not since suffered elevation or depression. The 
slopes ought to be gentle and well graded ; the valleys ought to be broad 
and waste-filled, and should truncate the underlying formations without 
reference to the attitude of the strata; and lava sheets ought to be 
almost undisturbed by the erosion which is rapidly dissecting into buttes 
the similar formations of the block upheaved by the second faulting. If, 
however, the block was depressed at the time of the last faulting, although 
we should expect to find these same topographic features, some of them 
would be drowned under new deposits of waste. If, on the other hand, 
the western block was somewhat elevated at the time of the faulting, 
we should look for a combination of the mature topography of the inter- 
fault cycle and the youthful topography of the present canyon cycle. 
The relative amounts of the two would depend largely on the amount of 
uplift, the lapse of time, and the texture of the strata. As a matter of 
fact, it is just such a combination of mature and young topography that 
we find all along the eastern border of the Basin Range province at the 
foot of the Hurricane and Grand Wash faults. 
For example, where the Virgin river, after crossing the Hurricane fault, 
emerges from its limestone canyon, it enters a region of which the sur- 
face is now basaltic lava. In this the river has cut a steep-sided trench 
a hundred feet deep. In the sides of this young canyon and to a greater 
extent in the sides of the similar young canyons of Ash and LeVerkin 
creeks, which here join it from the north, an old surface of sandstone is 
disclosed. The maturity or even old age of this surface is borne witness 
to by its extent as indicated by the breadth of the deposits of gravel and 
beds of cobbles which lie upon it and are in turn covered and preserved 
by lava. Still further evidence of its age is found in the fact that it 
truncates highly inclined strata without paying any attention to their 
attitude or relative resistance. In this it is like the rest of the Mohave 
peneplain, of which it forms the northern part. 
