NOTES ON GRAVITY DETERMINATIONS. 
65 
a discordance of .009 dyne (column 3a), and this is not re¬ 
duced by the geologic correction, but remains .009 dyne 
(column 6a). 
So far as the discussion of ten stations may warrant a 
general conclusion, that conclusion would appear to he that 
corrections based upon the densities of the accessible rocks 
may be applied with advantage to observations discussed 
under the theory of high rigidity, but not to observations 
discussed under the theory of isostasy. It is well to hold 
lightly a generalization from so small a number of particu¬ 
lars, but this one is strengthened by certain theoretic con¬ 
siderations. Under the isostatic theory the mass of each 
unit column of earth matter is approximately the same, dif¬ 
ferences of mean density being compensated by differences 
of altitude, and variations from the mean in one part of the 
column being compensated by opposite variations in other 
parts. Allowance for the density peculiarities of a part 
should not therefore be made. Under the theory of high 
rigidity there is no compensatory adjustment, and correction 
may properly be made for any local peculiarity which is 
discovered. 
2. Bearing of the Observations on the Theory of Isostasy. 
In view of the prospect that the present chain of stations 
will soon be expanded into a system, a comprehensive dis¬ 
cussion of the results would be premature, but there may be 
advantage in considering their general bearing. Prelimi¬ 
nary discussion is an important part of the interaction of 
theory and observation, and if the general tendency of pres¬ 
ent results can be discovered, the selection of future stations 
may be more effective. 
Let us postulate that the greatest features of the earth’s 
relief, such as continents and great plateaus, are sustained 
isostatieally, and that the small features, such as hills and 
small mountains, are within the competence of terrestrial 
rigidity, and then let us inquire what the pendulum work 
of the Coast Survey has to tell of the status of features of 
