92 
DYNAMICAL GEOLOGY 
trophic movement, there is regional upraising the upper part of 
the local prism which is elevated and eroded until the old pene¬ 
plain again reaches sky and a level perhaps of two or three miles 
above sea-level, the lower section appears as a schistose mass. 
Such seems to be the very thing which happens in the Rocky 
Mountains, the Piedmont Plateau and many other places. 
According to mathematical equation established or assumed the 
depth to which crustal compensation acts is about seventy miles. 
Geologic interest does not extend nearly so far. At a depth of 
five or six miles all rocks are crushed and facile rock-flowage is 
attained. The mobility of a rock-mass and the recrystallization 
along movement planes such as develops schistosity, surely is as 
perfect at six or seven miles as it is at seventy, or seven hundred 
miles. 
This very thing appears to have transpired in some of the cor- 
dilleran regions. In the Piedmont Plateau Paleozoic sedimentation 
takes place on a sinking tract until 30,00 to 40,000 feet of deposits 
accumulate. The great pre-Cambrian section of sediments, the 
Cambric, Orovicic, and Siluric piles are carried down quite into 
the zone of rock-flowage. When they return to sky again the 
old Paleozoics appear as schists, which until very lately are sup¬ 
posed to be Azoic in age and among the most ancient rocks on 
our continent. 
In the southern Rockies the course of events is similar, only 
that it is repeated again and again. Here, curiously, the geologic 
conclusion on diastrophic movements is that the latter directly 
contradict at every stage the essential demands of isostasy. This, 
however, is not necessarily so. Considering the zone of rock 
fracture as a geometric solid without reference to its geologic 
structure, it makes little difference to what depth a given peneplain 
be depressed. Even with two to four miles of sediments resting 
upon this old land surface a rock prism over each particular 
area remains isostatically balanced. In a depressed block a con¬ 
siderable part beneath the level of the buried peneplain long since 
flows off and loses its sedimentary identity. In both cases the 
geodetic datum is a fixed one. Not so with the geologic directrix; 
for the latter is a variable quantity, and at no time or point is it 
coincident with sea-level. 
On the other hand an upraised peneplain is followed closely 
