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HUNTINGTON AND GOLDTHWAIT: THE HURRICANE FAULT. 227 
erosion begun while the first faulting was in progress was carried forward 
to such an extent that the topography assumed a thoroughly mature 
character or even reached old age. The original topography due to 
faulting and folding was so far effaced that the valleys of the main 
streams were reduced nearly to base-level, forming the Mohavé pene- 
plain. In the more remote regions there were low rounded mountains 
with well-graded slopes, and the Pine Valley mountains had almost as 
great relief as at present. Differences in hardness of strata produced 
practically no effect in the most completely base-levelled regions. Even 
in less eroded places erosion had goné so far that a fault scarp several 
thousand feet high had been worn away and even reversed, so that the 
hard strata of the downthrown side stood higher than the softer ones of 
the uplifted side. Near the end of the period there was considerable 
aggradation in many valleys, and numerous volcanoes poured out large 
sheets of basaltic lava. 
The evidence of this long inter-fault cycle is found in facts of five 
classes which will be more fully discussed in the succeeding paragraphs. 
(1) The much greater northward recession of transverse cliffs on the 
upheaved than on the downthrown side of the faults demands a long 
lapse of time since the first faulting. (2) Many maturely dissected or 
even base-levelled surfaces belonging to the ancient topography have 
been buried by lava flows, and are now exposed in cross-section in the 
walls of young canyons or in the scarps of recent faults. (3) Portions 
of the lofty plateaus lying at a distance from their borders exhibit an 
upland surface of subdued graded slopes and broad valleys utterly dif- 
ferent from the precipitous youthful slopes of the peripheral regions. 
This surface seems to be a part of the mature topography of the previous, 
or inter-fault cycle that has not yet heen effected by the renewed erosion 
of the present post-fault, or canyon cycle. (4) At Kanarra the old 
Hurricane fault is crossed by the new one running almost parallel to it. 
The contrast between the mature topography of the one and the young 
topography of the other indicates a great difference in age. (5) During 
the latter part of the inter-fault cycle many valleys where erosion is now 
active were areas of deposition. The surface on which these deposits lie 
shows broad flat valleys the floors of which truncate highly inclined 
strata without reference to their attitude or texture. Corresponding to 
the valley floors are gently sloping grade plains, the sides of the ancient 
valleys, which lie at a higher level and are now being undercut. These 
two types of surface seem to indicate a previous topography far more 
mature than the present. 
