NOTES ON GRAVITY DETERMINATIONS. 
73 
later Mesozoic time; east of it there has been much sedi¬ 
mentation. The stations stand in a transition zone where 
degradation has been followed by moderate sedimentation. 
Here again the excess of matter is not explained. 
In view of the magnitude of the deviations from isostasy 
thus developed, the question may pertinently be raised 
whether the isostatic theory is more advantageous than the 
theory of high rigidity as a basis for the discussion of the 
observations. Though this question is practically answered 
by a comparison of the reduced values Mr. Putnam has 
tabulated and illustrated, there is some reason for adding 
in this connection the result of a partially independent com¬ 
putation. As already stated in discussing the geologic cor¬ 
rection, the theory of high rigidity is represented in the 
reduction by the correction for the attraction of material 
above sealevel. The isostatic theory is represented by the 
omission of that correction and the addition of the correc¬ 
tion for reduction to mean plain. Taking the 26 stations 
for which the mean plain correction has been computed and 
completing the reduction under the isostatic theory, I then 
subtracted the mean of the measurements from the individ¬ 
ual measurements and found an average residual of .026 
dyne. Treating the same measurements according to the 
theory of high rigidity and employing the same procedure 
in other respects, I obtained an average residual of .156 
dyne. The extreme range in the first case is .093 dyne ; in 
the second, .591 dyne. The measurements are thus seen to 
be six times as discordant from the point of view of rigidity 
as they are from the point of view of isostasy. 
Summary .—The measurements of gravity appear far more 
harmonious when the method of reduction postulates isos- 
tacy than when it postulates high rigidity. Nearly all the 
local peculiarities of gravity admit of simple and rational 
explanation on the theory that the continent as a whole is 
approximately isostatic, and that the interior plain is almost 
perfectly isostatic. Most of the deviations from the normal 
arise from excess of matter and are associated with uplift. 
