28 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Hurricane ledge. The Permian beds stand, however, opposite to the 
lower and upper Aubrey beds in the Uinkaret block. The Hurricane 
fault in this locality must therefore pass underneath the lava bed, and 
its throw must be at least a thousand feet. The effect of faulting, with- 
out erosion, is shown in the back block of Figure 8. At a later time when 
the lava was erupted, erosion of the faulted mass must have progressed 
so far that the Permian clays had been largely removed from the Uin- 
karet block hereabouts, while they still largely remained on the Shiv- 
wits, so as to make a level surface across the fault line on which the lava 
Ficure 8. 
Diagrams to illustrate the history of the Hurricane fauit near Coal spring. The back block 
shows the effect of faulting without erosion. The middle block shows a volcano erupted 
near the fault after the faulted mass had been worn down to a peneplain. The front 
block gives a generalized view of existing conditions produced by uplift and erosion of 
the middle block. 
was poured out, as in the middle block of Figure 8. It is evident that 
such a correspondence of level across the fault on beds of very unequal 
resistance could have been produced only by long-continued and wide- 
spread erosion when the region stood much lower than now. ‘This 
structure may therefore be taken as another point in the evidence that: 
demands two cycles of erosion (the plateau cycle and the canyon cycle) 
for the physiographic development of the region. The chief movement 
of the Hurricane fault must consequently be dated far back in the plateau 
