I 
62 GILBERT. 
the formations and the influence of their peculiarities of 
density on the local attraction. Special attention was given 
also to the records of deep borings, and beyond the depths 
penetrated by borings the characters of the sedimentary 
rocks were afterwards inferred from a variety of geologic 
data, gathered, chiefly by others, in localities more or less 
remote. The results may in each case be regarded as rough 
approximations, but they serve fairly well the purpose of 
this inquiry, which was to determine whether a correction 
to the observed gravity may advantageously be based on 
such information as the geologist can adduce as to the 
density of the subjacent rocks. 
In Table I column 2 gives the observed value of gravity, 
in dynes, after correction for latitude, altitude, topography, 
and the attraction of matter between the surface at the sta¬ 
tion and sealevel beneath it. The latitude correction was 
a reduction to 40°; the altitude correction, a reduction to 
sealevel. The topographic correction, in effect, substitutes 
a level plain for the diversified topography about the sta¬ 
tion.* The correction for the attraction of matter above sea¬ 
level recognizes the theory of high rigidity, and in the treat¬ 
ment of observations distinguishes that theory from the 
theory of isostasy. It gives, indeed, an imperfect recogni¬ 
tion, because under that theory the sealevel, or geoid of ref¬ 
erence, rises in continental regions above the spheroid of 
reference, thus making the apparent altitude too small, but 
it is probable that this imperfection does not affect the quali¬ 
tative results of the present inquiry. 
Column 3 gives the same values as column 2, except that 
the correction for the attraction of matter above sealevel was 
omitted. As compared with column 2, it represents the 
theory of isostasy. 
Column 4 gives the geologic correction. At each station 
the surface rocks are sedimentary and have small dip. The 
sedimentaries vary in thickness from one-half mile to three 
* The nature of the topographic correction is described by Mr. Putnam 
on pages 43^16. I employed the values computed by him. 
