NOTES ON THE GRAVITY DETERMINATIONS 
REPORTED BY MR. G. R. PUTNAM 
BY 
Grove Karl Gilbert 
[Read before the Society, March 16, 1895, and published by permission 
of the Director of the U. S. Geological Survey.] 
1. Corrections based on the Local Geology. 
By some physicists it is thought probable that the mate¬ 
rial of the earth’s crust is highly rigid, and that by this 
rigidity the continents are upheld. By others it is thought 
more probable that the earth’s crust is somewhat plastic, and 
that the continents stand high because their material is light. 
The question of high rigidity versus isostasy affects the 
theories of geologists, and hence they are interested in the 
diagnosis of the crust by means of the pendulum. Ameri¬ 
can geologists have reason to be specially interested in the 
recent pendulum work of the Coast Survey, not only because 
the efficiency of the new apparatus enables it to multiply 
accurate measurements with unprecedented rapidity, but 
because the results are related to geologic provinces .with 
whose structure we are acquainted. Already much light 
has been thrown on the problem of crustal rigidity, and it 
is hardly rash to look forward to a satisfactory solution if 
the work is continued for a few years with equal energy 
and skill. 
Last autumn I followed in Mr. Putnam’s tracks so far as 
to visit ten of his stations, namely, Colorado Springs, Denver, 
Wallace, Ellsworth, Kansas City, St. Louis, Terre Haute, 
Chicago, Cincinnati, and Cleveland. At each station I ex¬ 
amined the local geology with reference to the density of 
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