102 
ISOSTATIC THEORY 
Duttonian hypothesis demands. They allow themselves to be 
worn down to the very level of the sea without appreciable attempt 
at upward movement. After reaching that level when their 
positive volume had been completely consumed, the entire tract 
which lofty mountains had occupied proceeds to sink further. 
Downward movement continues until the old sea-level plain attains 
a position more than two miles beneath the surface of the epi-con- 
tinental ocean. Despite the removal of its former great positive 
load the net result of regional vertical movement is notably 
negative. 
For four successive geographic cycles similar procedure takes 
place, so that the Comanchan episode is not a solitary instance. 
Removal of load through erosion is not accompanied by progressive 
upraising. Rolling of oceanic waters over the tract follows 
regional unloading. Maximum sedimental loading of the area is 
immediately succeeded by extensive uplifting. At every stage of 
diastrophic activity phenomena are met that are diametrically 
opposed to those which Dutton postulates. Isostatic hypothesis 
in this instance utterly fails to sustain itself before such an array 
of critical testimony. Geologically the crustal span of isostatic 
flotation seems not to hold for cordilleran tracts the width of the 
state of Colorado. Orogeny manifestly rests on other telluric 
forces. 
Decisive and convincing as are the recent geological observa¬ 
tions bearing upon specific aspects of isostasy it is mining that 
gives early clue and first points out the lines of determined attack. 
Local geographic distribution of the mines first shows that the 
crustal span the breadth of the Rockies is as inadequate isostaticly 
as is that of the narrow desert range of the Great Basin. ^ 
Scarcity of mines along the lines which hypothesis, if true, de¬ 
manded at once outlines the mode of solving the problem. It 
seems passing strange that practical and commercial data should 
so long antedate the strictly theoretical and scientific. We do not 
always realize how closely after all the theoretical and practical are 
intertwined. Perhaps the isostatic measurement of a compensating 
crustal span might have been accomplished just as well without 
first aid from mining. The fact remains that it did not. It may 
be well doubted that it ever would. 
While today the general hypothesis of isostasy in the strictly 
Duttonian sense is steadily losing ground among geologists, it 
