DYNAMICAL GEOLOGY 
93 
by an uprising of the upper limit of the former zone of rock- 
flow. In this instance the structureless geometric block and the.i 
stratified prism really coincide. Yet, the mathematical datum 
still remains at sea-level and the lower portion of the geometric 
block really passes into the zone of previous rock-flowage. 
No mountainous tract, perhaps, is so especially well adapted 
for test of diastrophic movement, or for determination of the span 
of isostatic adjustment as the Rocky Mountains. This noble cor¬ 
dillera occupies a broad belt, which, from the earliest geological 
times, has undergone almost continuous oscillation. The area is 
one which has been repeatedly uplifted. It is one which has 
suffered again and again extensive planation. Its orographic 
change is never so great as entirely to obliterate the salient fea¬ 
tures of the different movements. Its later upraisings and down- 
sinkings are exceptionally well marked. The geologic record 
of events is unusually full and clear. 
Although from very earliest eons the southern Rocky Mountain 
tract experiences repeated upliftings those taking place since the 
close of Paleozoic times are most remarkable. The latter are 
four in number. There are a 'like number of total effacements. 
Each of the ancestral Rockies appears to have been no less majes¬ 
tic than are the Rocky Mountains of today. Each of the old 
levelings is down to the surface of the sea. Peneplanation in 
each instance is as perfect as is probably ever attained. Great 
planation epochs mark Comanchan, Laramian, Miocene and Re- 
. cent times. 
Both local and regional loading and unloading through sedi¬ 
mentation and erosion are strictly problems in mechanics. Ac¬ 
cording to the fundamental premise of the isostatic hypothesis 
mountains continually rise because of the removal of their sum¬ 
mits by erosion, thus lightening the local crustal load. On the 
' other hand, depending upon the argument of the astronomer 
Hershel, areas of sedimentation are tracts of notable insinking of 
the crust. 
Singularly, the Jurassic ancestral Rockies appear not to have 
grown higher as their substance wasted away. Without sign of 
struggle they allow themselves to be worn down to the very level 
of the sea. After reaching sea-level, when all their positive vol¬ 
ume vanishes, the tract which the Rocky Mountains once occu- 
