106 Adams and Colcer — Elastic Constants of Rocks. 



isotropic material in which the elasticity is equal in all direc- 

 tions, are greater than in the Tennessee marble, which is rather 

 coarse in grain, and in Baveno granite, which is the coarsest 

 rock of the set. The greatest differences obtained are those 

 found in the linest-grained rock in the series, viz., the Sudbury 

 diabase. 



It is evident therefore that the different moduli of elasticity 

 of the constituent grains of a rock do not introduce any per- 

 ceptible error in measurements made by this method, when a 

 column an inch in diameter is employed, and when the rocks 

 are not coarser in grain than the Baveno granite. In fact, 

 when surrounded on all sides by other grains, no individual 

 grain can expand freely, as it would if subjected to compres- 

 sion unhampered by any surrounding medium, and thus the 

 anisotropic character of the individual grains produces but little 

 effect on the elasticity of the rock as a whole. 



These experiments also show that in the case of rocks com- 

 posed of several minerals, it makes no perceptible difference 

 whether the points of attachment of the instrument are em- 

 bedded in the grains of one mineral or of another. The chief 

 source of error, and the one to which the variations observed 

 are for the most part to be attributed, seems to be a mechani- 

 cal one, viz., the difficulty of getting an ideal contact between 

 these points of attachment and the specimen, especially in 

 view of the extremely small dimensions of the movement to be 

 measured. 



The question of the influence of temperature on the elasti- 

 city and compressibility of rocks is of course one which has 

 an important bearing on certain problems of geophysics. The 

 only investigation of this subject, so far as can be ascertained, 

 consists of a few preliminary experiments by Nagaoka and 

 Kusakabe.* In these the torsion method was employed, and 

 the experiments were carried out on a single rock, viz., sand- 

 stone. This rock, as has already been mentioned, being porous 

 and stratified in character, is a material whose elastic proper- 

 ties are far from ideal. The results are summed up by the 

 authors in the following words: " Preliminary experiments 

 with sandstone show that the modulus of elasticity is much 

 affected by the variation of temperature, i. e., about 0*5 per 

 cent, per degree. It does not, however, necessarily diminish 

 with the increase of temperature where the temperature is low, 

 i. e., it is maximum about 9° C." 



As has been shown, however, the values for the elastic con- 

 stants obtained by this torsion and bending method have 

 yielded results which cannot in all cases be correct, and which 

 differ very considerably from those obtained by the much more 



* Modulus of Elasticity of Bocks, and Velocities of Seismic Waves, Publi- 

 cations of the Earthquake Investigation Committee, No. 17, Tokyo, 1904, 

 p. 43. 



