ELASTIC CONSTANTS OF ROCKS- 13 
If we call A the cross-sectional area of the specimen when stressed by a 
load, P, and x the decrease of length over a measured length, L, gripped 
between the screw points of the measuring apparatus, we obtain 
E- PL 
~^A 
which, in case of a specimen of wrought iron examined for a range of 8,000 
pounds, gave a value of 28,100,000, the units being pounds and inches. 
The ratio m of the longitudinal strain to the lateral strain in the same case 
was 3.65, and using the formula 
D-I 
m 
3m- 2 
we obtain for the modulus of cubical compression (or bulk modulus) D, the 
value 21,300,000, a constant for the material, the reciprocal of which gives 
the decrease in volume of i cubic inch for i pound of pressure. 
While certain rocks, such as many of the marbles, have a structure identical 
with that of wrought iron, most of the rocks constituting the earth's crust 
are composed of several minerals, and thus resemble cast iron in character^ 
the gray variety of this substance being an aggregate of crystals or individ- 
uals of the metal iron (wrought iron), graphite, etc. 
It will therefore be of interest to ascertain how a specimen of cast iron be- 
haves under compression stress, and how far its elasticity falls short of that 
which would be exhibited by a perfectly elastic body. 
For this purpose a fine-grained specimen of somewhat hard cast iron was 
faced and tested. The results of this test are given in the following table, 
and the stress-strain curves are plotted in figure 3 . I represents longitudinal 
compression and II lateral extension. 
The behavior of cast iron, as exhibited by these experimental results, shows 
a falling away from the theoretical standard of perfect elasticity, but even in 
the most perfectly elastic bodies there is probably a slight hysteresis effect, 
so that we are justified in using the results obtained to calculate the mod- 
ulus of compressibility, if the error introduced thereby is negligible or very 
small. 
It may be pointed out that this method and others of the indirect type have 
been freely used to obtain values of the bulk modulus for cast iron and metals 
of like character, and it will be shown that the composite crystalline rocks are 
very similar to cast iron in their behavior under stress, although generally 
more perfectly elastic. 
