ELASTIC CONSTANTS OF ROCKS- 15 
APPLICATION OF THE METHOD OF SIMPLE COMPRESSION TO THE 
DETERMINATION OF THE COMPRESSIBILITY OF ROCKS. 
It has been noted in the case of marble when subjected to bending stress 
that the strain as exhibited by the deflection of a point of the bar increases with 
the time, and the strength under shear produced by a torque was also found to 
be so small that a determination of the strain was very difficult to measure. 
These difficulties have been noted by Nagaoka,* who states that: 
Preliminary experiments on granite show that Hooke'slaw does not hold even for very 
small flexure and tension, and that the after effect is very considerable from the pressure, 
when the prism is sufficiently loaded or twisted, the deviation from the direct proportion- 
ality between strain and stress was incomparably great as compared with that observed 
in metal. This must be chiefly due to the low limit of elasticity, so that it is necessary 
to experiment only within very narrow limits of loading and twisting. These limits are 
widely different for different specimens of rocks, and the modulus of elasticity, as well as 
that of rigidity, was always determined with such stresses as will approximately produce 
strains proportional to them. The deviation from Hooke's law was prominent in certain 
specimens of sandstone, and it was more marked in tension than in flexure experiments; 
in certain rocks it is indeed doubtful if anything like a proportionality between stress 
and strain can be found, even for extremely small change of shape. On releasing these 
rocks from stress the return toward the former state is extremely small, showing that the 
elasticity of the rock is of a very inferior order. 
These observations of Nagaoka for bending and twisting have been con- 
firmed by our own deflection experiments, as above mentioned. 
If, however, the rock be subjected to direct compression, strains in which 
the time effect is small and the lag of the strain is also small are almost inyari- 
ably obtained. This is especially the case if before the actual experiment is 
carried out the material be several times subjected to a range of stresses at 
least as great as those employed in the experiment itself. This preliminary 
stressing brings the material to "a state of ease," and is also commonly 
adopted when the elastic constants of metals are determined. 
It is evident, therefore, that this direct compression method may with con- 
fidence be applied to the measurement of the cubic compression of rocks, 
although as mentioned below the accuracy of the result so obtained will differ 
with different classes of rocks. 
If the rock be massive, compact, and crystalline (or glassy) the method can 
be safely employed and good results will be obtained. If, on the other hand, 
the rock is schistose, porous, or loosely coherent, the method will from the 
nature of the case be very much less satisfactory. 
The plutonic igneous rocks as a class most nearly resemble the metals 
in structure, being holocrystalline and massive, and therefore present the 
*Elastic Constants of Rocks and the Velocity of Seismic Waves. H. Nagaoka. Phil. 
Mag., vol. L, 1906, p. 58. 
