DYNAMICAL GEOLOGY 
91 
fail even for great cordilleran tracts the width of the State of 
Colorado a pointed question at once arises as to what is really 
the geological datum plane from which calculations are started 
and to which conclusions are referred. 
Both geodesist and geologist starts his problem at sea-level. 
While the first mentioned throughout his calculations retains that 
datum plane, the other soon wanders away from it and gives first 
reference place to a plane which he can clearly follow in the field. 
In the last analysis the geological directrix turns out to be some 
regional erosion plain of the first order. Among more recent 
instances it is a peneplain; among the more ancient cases it is 
a great plane of unconformity. The relation to sea-level of such 
plane more or less extensively deformed though it subsequently 
becomes, gives the geologist his chief clue to the evaluation of di- 
astrophic movement. All unconsciously, perhaps, he extends his 
diastrophic estimates to isostatic problems. With his engineering 
bias of sedimental loading and erosional unloading of local tracts 
of the earth’s crust he naturally and quickly reaches some old 
peneplain as a basis of his figuring. 
Starting at sea-level, or more properly at a hypothetical level 
about two miles below, the geodetic calculations treat the datum 
as a constant and the crust as an amorphous, geometric solid of 
great and indefinite thickness. With the same starting point the 
peneplain becomes a variable factor; and the crustal prism pos- 
sesses a straticulate structure susceptible of recording the slightest 
deformations and the smallest vertical movements. The two dat¬ 
um planes not only do not always lie together, but they are usually 
quite distinct, and soon may become miles apart. Their relative 
positions may vary between zero and infinity. Small wonder, 
then, that the geodetic and geologic conclusions are apparently so 
contradictory. 
Geodetic conception has the crustal prism, a rigid, structureless 
block, 70 miles or so in thickness, floating upon the mobile interior. 
Geology considers the same prism with varying thickness, only 
five or six miles in vertical measurement, and having the bottom 
of the old rock-section flowed off to form schists; the part of the 
section above the old peneplain being replaced by newer sedi¬ 
ments of specific gravity not so very different from that of the 
more ancient rocks. When, owing to a return swing of the dias- 
