GEOGRAPHY, TOPOGRAPHY, AND GENERAL GEOLOGY. 23 
ing and by erosional exhumation of the laccolith cores and surround- 
ing sediments, so far as these have been covered by the lavas, and 
developing a mature type of topography in the lavas themselves; 
(3) Hurricane faulting to the east, causing a slight renewal of activ- 
ity, exhibited in the sharp canyons and steep sides of some of the 
lava hills. 
There have thus apparently been three partial erosion* cycles, the 
second one now shown chiefly by the flat tops of mesas like the Har- 
mony Mountains and the third by the sharp ravines and canyons. 
But it is doubtful if the term cycle should be used here, for it 
implies uplift and renewal of activity of streams, the evidence for 
which is not satisfactory. There has been an interruption of erosion 
by the extrusion of the lavas, but the only evidence which can be 
cited for uplift and renewal are the sharp ravines and canyons; and 
it may be observed that both these and the grade slopes are being 
developed at the present time. The rapid development and modifi- 
cation of the grade slopes has already been referred to. But the 
rapid cutting of the ravines and canyons in these same grade slopes 
is no less striking. There is an alternate leveling up of the deep 
ravines to the grade slopes and the development of new ravines in 
periods of alternating high and low precipitation. 
Huntington and Goldthwait, in their excellent paper on the Toquer- 
ville area, cut by the' Hurricane fault to the southeast of the Iron 
Springs district, work out two cycbs of erosion similar to the second 
and third here presented. Their first cycle occurred after the extru- 
sion of trachyte, rhyolite, and andesite and subsequent faulting, and 
corresponds to the second cycle in the Iron Springs district. There 
appears to be no evidence in the Toquerville district for the prelava 
and postintrusion cycle of the Iron Springs district. The second 
cycle of Huntington and Goldthwait occurred after the basalt 
flows and the later Hurricane faulting. For this cycle there is scant 
evidence in the Iron Springs district. They cite the Pine Valley 
Mountains and similar structures of the Iron Springs district and 
southwest as an expression of the mature topography of their first 
cycle, and correlate it with certain remnants on the High Plateaus, 
which were then very much lower than at present, the Pine Valley 
Mountains being the dominant topographic feature. During the Hur- 
ricane faulting the High Plateaus were raised, and renewed erosion 
cut deep vertical-walled canyons into them, which are the present 
expression of the second cycle. This faulting produced very little 
difference of elevation west of the present Hurricane scarp, hence 
this region, including the Iron Springs district, shows mainly the old 
mature topography. There are, however, small canyons in the Iron 
Springs district which may show a renewal of activity and may he 
comparable to the steep-sided canyons of the High Plateau-. 
