332 THE JOURNAL OF GEOLOGY. 
The majority of the stations are arranged in a chain from 
the eastern coast to Salt Lake City. Two stations are on the 
Pacific Coast and three in Yellowstone Park. In discussing the 
measurements, I have started with the general postulate that 
continents and ocean beds are in isostatic equilibrium, and have 
sought to determine from the local values of gravity the extent 
to which various geological provinces of the country deviate 
from perfect isostatic adjustment. Between the Appalachian 
and Rocky Mountains is a great plain, which has been exempt 
for a succession of geologic periods from orogenic disturbances, 
and during that time has had exceptional opportunity for the 
gradual relief, through viscous flow, degradation-and sedimenta- 
tion, of the strains engendered by gravity in connection with 
anomalies of density. It seems, therefore, a priori probable that 
this plain is in approximate equilibrium; and, if so, the average 
attraction on the plain may advantageously be used as a stand- 
ard of reference in the consideration of other provinces. Eleven 
of the stations belong to the plain and they are well spaced from 
east to west. An examination shows the values of gravity at 
these stations to be notably accordant. When the mean of the 
eleven measurements is subtracted from the several measure- 
ments, the average residual is found to be only yapoa0 Of &, 
or such a differential acceleration as would be caused by the 
addition or subtraction of a layer of rock 240 feet thick. 
Referring all the measurements to the standard thus obtained, 
itis found that there is an excess of attraction in all the moun- 
tain districts where measurements were made. In the Rocky 
Mountains of Colorado there are two stations, at Pike’s Peak and 
Gunnison, and the excess of gravity determined at these stations 
is equivalent to the attraction of a rock layer 2200 feet thick. 
That is to say, if this mountain belt, 150 miles broad, were pared 
away to an average depth of 2200 feet, the local gravitation would 
then correspond to that on the interior plain. Now it also 
appears, as a generalization from the Hayden contour map of 
Colorado, that if this same district were leveled by removing the 
mountain tops and using the material to fill the valleys, it would 
