80 
DYNAMICAL GEOLOGY 
Great Basin it was fancied that so responsive was the earth’s 
crust to local loading and unloading through erosion the smallest 
mountain ridge or a single mountain was quickly and profoundly 
affected. The short, lofty, apparently tilted mountains were 
looked upon as huge fault-blocks floating upon the soft interior 
of the earth mobile almost as ice-cakes in a river at time of spring- 
break-up. This view of isostasy’s warmest supporters proved to 
be extreme and illusive. 
As, a generation later, this hypothesis began to undergo critical 
test for its verity it was found that the first extreme application 
was very far from holding. For such mountain masses as the 
Great Basin presented there appeared to be no marked recent 
movement discernible. Instead of being orographic blocks lately 
faulted and tilted the desert ranges proved to be really mountains 
of erosion. Their supposed fault-scarps were girdling effects of 
desert abrasion. No fault-lines characterized the mountain bases. 
The tectonics were relatively ancient. Whenever fault-line was 
discerned it was far out on the plain. Isostatic compensation sure¬ 
ly did not obtain in the case of narrow single mountain ranges. 
On a larger scale the cordillera of the Rocky Mountains is re¬ 
cently the theme of isostatic measurement by means strictly geo¬ 
logic. This tract seems to be exceptionally well suited to deter¬ 
mine the span of isostasy. From the earliest geologic times it 
undergoes continuous diastrophic oscillation. The area is one 
which has been repeatedly uplifted. It is one which suffers again 
and again extensive planation. With it orographic change is 
never so great as to obliterate entirely the salient features of the 
successive movements. Later upraisings and down-sinkings are 
especially well marked. The geologic record of events is un¬ 
usually full. 
Since the close of the Paleozoic era great planation periods mark 
Comanchan, Laramian, Miocene and Recent times. Comanchan 
peneplanation is particularly wide spread. Singularly, the Jur¬ 
assic ancestral Rockies do not appear to have grown higher as 
their substance wasted away, as isostasy demands. They allow 
themselves to be worn down to the very level of the sea. After 
reaching that level, when all their positive volume had vanished, 
the tract which the'mountains occupied proceeds to sink. At any 
rate depression is active until the old sea-level plain attains a 
