DYNAMICAL GEOLOGY 
95 
Isostasy of the geologist and isostasy of the mathematician are 
not, then, by any means the same thing. This is the basic reason 
why in the Rocky Cordilleran Region, as already emphasized, the 
geologic observations on isostasy are so directly out of harmony 
with geodetic demands. In following the only available datum 
plane from which to measure the amount of alleged isostatic flo¬ 
tation the geologist at once branches off from the fixed level of 
geodesy. Each group of conclusions are thus necessarily anti¬ 
thetical. The geological directrix of isostasy resolves itself into 
an old erosional plain, or peneplain as the geomorphologists are 
pleased to call it. In all considerations of isostatic measurement 
this distinction is obviously fundamental. In all the elaborate 
calculations of mathematician the latter does not really touch the 
conception of geologist. Although the geological directrix of iso¬ 
stasy is the one factor of all upon which geodetic hypothesis 
depends yet it appears to be without representation in mathematical 
equation. ’ Keyes. 
Geotectonic Economy of Thrust-faulting. Critical analysis of 
the larger geologic relations of normal and reversed faults empha¬ 
sizes the fact that it is illogical to consider them together, as if 
they belonged genetically to the same order of structural phenom¬ 
ena. The one being merely limited crustal adjustment through 
gravity presents marked contrast to the other which appears to be 
the direct expression of cosmical stress, and therefore in so far as 
the earth is concerned, of universal causes. 
That thrust-faults occur more frequently in the hard or brittle 
formations than elsewhere, that they are mainly confined to the 
very old rocks rather than to the younger terranes, that they be 
come more prominent, more important and more numerous in the 
bottom of the earth’s crust, or zone of rock-fracture, than near 
the top, and that under ordinary conditions they rarely reach sky 
except through chance exposure by profound local erosion, are 
generalizations of great significance but attract small notice and 
are little associated genetically. 
This broad or universal cause appears to be found in the re¬ 
tardation of the earth’s rotation, producing repeated and cumula¬ 
tive displacement of the earth’s radius of molar repose, or radial 
line of no strain. Keyes. 
