ELASTIC CONSTANTS OF ROCKS. 
are anisotropic, that is, they have different moduli of elasticity in different 
directions. In massive rocks such as those investigated, however, these grains 
occur in the rock with an absolutely irregular orientation and would in the 
case of a fine-grained rock mutually compensate for one another in any trans- 
verse line along which the expansion of the rock under compression might 
be measured. If, however, the rock were coarser in grain, fewer individual 
crystals would be found in any transverse line of section, and there might 
possibly in this way be a lack of compensation, as the rock in one section 
might be composed of grains whose axis of greater elasticity approximated 
on an average more nearly to the direction of measurement than in other 
sections. If such were really the case, there should be in these coarser- 
grained rocks an exceptionally great variation in the readings obtained from 
different specimens of the same rock, as well as from the different sections in 
the same specimen. 
But such is not the case, as will be seen by an examination of the figures in 
accompanying table. They represent the results obtained from ten measure- 
ments of the compressibility of Baveno granite, which is coarse in grain, and 
ten of Sudbury diabase, which is very fine in grain, together with eight 
measurements on Tennessee limestone, which is rather coarse grain, and seven 
on plate glass. They were made in each case on two or more specimens cut 
from the same mass and the measurements of the expansion were made on 
several different planes through each, so that in every case the measurement 
was effected in a different line through the rock, all of these, however, of 
course being at right angles to the direction of the compressive stress and 
lying in the medial plane of the column. 
Full details concerning each measurement will be found in the tables 
which set forth the results obtained, under the sections dealing with the several 
rocks in question. The size of grain and the texture of the rock can also be 
seen by examining the photomicrographs and color prints of the polished 
surfaces of the respective rocks. 
Max. 
Min. 
Diff. 
Baveno granite (coarse) 10 trials 
Sudbury diabase (very fine) 10 trials. . . . 
Plate glass, 13 trials. . 
4,880,000 
11,170,000 
6,930,000 
4,380,000 
9,655,000 
6,020 ooo 
500,000 
1,515,000 
910 ooo 
Tennessee marble (rather coarse) 7 trials . 
6,130,000 
5,770,000 
360,000 
It will thus be seen that there is no correspondence between the coarse- 
ness of grain and the magnitude of the variations in the readings obtained. 
The differences in glass, which is an iso tropic material in which the elasticity 
is equal in all directions, are greater than in the Tennessee marble, which is 
rather coarse in grain, and in Baveno granite, which is the coarsest rock of 
