NOTES ON GRAVITY DETERMINATIONS. 
71 
of 400 rock-feet. This discrepancy of 1,800 rock-feet is not 
accounted for by any known facts of local structure. The 
stations rest on the same geological formation and are simi¬ 
larly related to broad arches of strata. If we assume that 
the area of the mean plain of reference is much too small, 
and that the broad district behaves as a unit, the difference 
in the general degradation about the two stations accounts 
for two-thirds of the discrepancy. The mean plain of a cir¬ 
cular tract with 100 miles radius and including both sta¬ 
tions has an estimated altitude of 6,000 feet, and reference to 
this gives the Grand Junction station an excess of 800 rock- 
feet and the Green River station an excess of 200 rock-feet. 
These are not more discordant than some of the stations of 
the interior plain. 
The Wasatch plateau in the vicinity of the Pleasant Valley 
gravity station is 50 miles wide and may be called a single, 
broad, low corrugation. Its average height above adjacent 
lowlands is 1,500 to 2,000 feet. The gravity station is in ail 
eroded valley near its eastern edge, and the measurement 
indicates an excess of 1,300 rock-feet. 
From the Wasatch plateau and its northward continua¬ 
tion, the Wasatch range, westward to the Sierra Nevada 
stretches the province of the Desert ranges, 450 miles broad. 
It is a corrugated plateau, and differs from the other prov¬ 
inces in that it loses no material by degradation. The waste 
from its ridges is stored in its valley troughs. If the ridges 
rise because light and the troughs sink because heavy, then, 
as degradation unloads the ridges and loads the troughs, 
whatever lag there may be between cause and effect should 
find expression in a defect of gravity on the ridges and an 
excess of gravity in the troughs. The Salt Lake City sta¬ 
tion stands on the alluvial load of a trough near the base of 
the Wasatch range. Assuming trough and ridge to have 
the isostatic relation outlined, the proper plain of reference 
is the mean plain of the trough, and that has about the same 
altitude as the station. This assumption (a, Table III) yields 
a small defect of gravity instead of the excess theoretically 
anticipated. 
10—Bull. Phil. Soc., Wash., Vol. 13. 
