66 
GILBERT. 
intermediate size, namely, the greater mountains and smaller 
plateaus. 
Between the Rocky and Appalachian mountains stretches 
a vast plain, 1,200 miles in its smallest dimension. For at 
least five of the great periods of geologic chronology it has 
been exempt from orogenic corrugation, and the topographic 
evidences of earlier corrugation have been practically oblit¬ 
erated. Here, if anywhere on the continent, isostatic equi¬ 
librium should be established. For this reason I have taken 
the average gravity of this region as normal and used it as 
a standard of comparison. Eleven stations fall within the 
district—the ten enumerated above and Ithaca, New York. 
Before taking the mean of the values at these stations, one 
more adjustment was made—the adjustment to the mean 
plain. The topographic correction adjusts the value for 
gravity to the hypothetic condition of the surface which 
would result if the neighboring hills and mountains were 
removed down to the level of the station and the neighbor¬ 
ing valleys were filled up to the same level. Ordinarily this 
implies a change in the quantity of matter and a consequent 
disturbance of isostatic equilibrium. If instead of this we 
conceive the mountains to be graded down to such a level 
that the removed material exactly fills the valleys, we hypo¬ 
thetically leave the isostatic relations undisturbed. It is 
manifest that in a discussion like the present, which postu¬ 
lates general isostasy and seeks to learn the limitations of 
isostasy, assumptions falsifying the regional load should be 
avoided if possible. To adjust the values from the hypo¬ 
thetic plain at the level of the station to the hypothetic 
mean plain it is necessary to add or subtract the attraction 
of a plate of rock as thick as the space between the two 
plains. When the station is below the mean plain the cor¬ 
rection is additive; when above, subtractive. 
The amount of the correction and sometimes its sign 
depend on the extent of the district averaged to determine 
the height of the mean plain. The diameter of the proper 
district is a function of the rigidity of the crust and requires 
