232 ° BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
them and from any of the cliffs that are due to recent faulting. These 
slopes have been renewed and steepened by recent erosion, but neverthe- 
less they are continuous with the surface under the lava and seem to be > 
the product of the same period of erosion. If our interpretation is 
correct, the mature topography at the top of the escarpment is a battered 
remnant of that which prevailed before the plateau region was uplifted 
at the time of the last great faulting. West of the fault where the main 
body of the lava sheet now lies, soft sandstones and shales had allowed 
the reduction of the surface nearly to base-level ; east of the fault rela- 
tively hard limestone produced a gentle escarpment rising to a plateau 
three or four hundred feet higher than the adjacent lowland, a Monad- 
nock of Carboniferous strata in the Mohave peneplain. 
A more perfectly preserved example of the ancient topography is dis- 
played in the lofty plateau of Colob, a few miles beyond the northeastern 
corner of the area shown on our map. This forms a portion of the up- 
heaved block east of the Hurricane fault, and lies at an elevation of from 
eight thousand to ten thousand feet. On the west it is bounded by a 
great escarpment, due in part to the old fault and in part to the new 
one, as will be explained later. To the south it is limited by the impos- 
ing Vermilion cliffs, nearly two thousand feet high, which here reach 
their greatest development. To the north and east it stretches away 
indefinitely, until it merges into other portions of Dutton’s High plat- 
eaus, of which it may be considered a fair sample. Around the edges 
of ‘‘ Colob,” as it is locally called, the revived activity of erosion conse- 
quent on the last faulting has caused most energetic dissection, and 
steep cliffs, naked ledges, and profound canyons are the rule. In the 
centre of the plateau, however, the ancient pre-faulting topography is 
still preserved in broad vistas of gently sloping hills and dales, the finest 
of summer pasture for sheep and cattle. Sometimes for miles scarcely 
an outcrop appears. The relief, to be sure, is considerable, amounting 
to fully two thousand feet, and in certain places the hills might well be 
called mountains if that term could be correctly applied to an elevation 
having a distinctly non-mountainons structure of nearly horizontal 
strata. The maturity of the topography and its contrast to the youth- 
ful character of the peripheral regions are so marked that we seem 
forced to believe that it is the product, not of the present canyon cycle, 
but of the previous inter-fault cycle. 
(4) Tae New anp THE Oxtp Fautts at Kanarra. The escarp- 
ment which bounds the Colob plateau on the west shows the old fault 
and the new exposed under conditions where it is easy to compare their 
