228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(1) Recession or Ciirrs. In their present position the strata of the 
central and western part of the Plateau province have a gentle but very 
persistent dip to the north. It is often locally complicated by dips to 
the east due to faulting or by the eastward dip of the old monoclines. 
Its persistence and general uniformity of direction, however, find ex- 
pression in the long lines of south-facing cliffs which cross the country 
at right angles to the dip. These are due to the alternation of hard and 
soft strata. As the later are continually worn away, the harder beds 
that overlie them are left without support and fall as talus, causing the 
cliffs to retreat continually northward down the dip. The hard talus 
that falls from the cliffs gradually piles up at their base and checks their 
rate of retreat. The amount of this checking depends on the rapidity 
with which material is carried away from the foot of the cliffs, and this 
in turn depends on elevation above base-level. Hence, ~here faults 
cut the cliffs at right angles, as the north and south faults cut the 
cliffs in the Plateau province, the cliffs on the heaved side, being at a 
greater elevation, are more fully exposed to the effects of erosion and 
retreat faster than those on the low-lying downthrown side. Powell 
(4, p. 191), Gilbert (a, p. 51), and Dutton (¢, p. 200) all recognize this, 
but, as Davis has pointed out (0, p. 144), “they do not explicitly con- 
nect the amount of recession with the date of faulting.” Moreover, as 
the same writer has shown, the present interval between the correspond- 
ing cliffs on the two sides of a fault represents not merely the amount 
of recession since the faulting occurred, but the excess of the retreat on 
the heaved side over that on the thrown side. 
Near Toquerville the cliffs formed by the Kanab sandstone are fifteen 
miles farther north on the eastern or uplifted side of the Hurricane 
fault than on the western side. The first faulting must have taken 
place so long ago that the upper cliffs have had time to retreat fifteen 
miles farther than their counterparts on the thrown block. On the 
other hand, formations which are separated only by the later faulting 
match very closely on the two sides of the line of displacement, indi- 
cating that this took place quite recently. Between the two periods of 
faulting there must have intervened a long inter-fault cycle of erosion. 
(2) Oxp SurFacEes covERED By Lava. Further evidence of the 
length of this inter-fault cycle is found in the old surfaces, portions of 
the ancient topography, preserved under recent lava flows, and now 
exposed in cross-section where the lava is cut by canyons or by the 
recent fault. Several of these have been already mentioned. It will be 
recalled that at Coal spring and at the mouth of the Virgin canyon lava 
