Adams and Coker — Elastic Constants of Rocks. 105 



method. In these two cases the results are Jess certain owing 

 to the greater hysteresis of the rock. 



It might at first sight appear that while the method em- 

 ployed is theoretically perfect as applied to the measurement 

 of the compressibility of vitreous rocks and of very fine-grained 

 crystalline rocks, a considerable error might be introduced when 

 the rocks are coarser in grain. In the case of all the common 

 crystalline rocks, the individual grains of which the rock is 

 composed 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 transverse 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 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 excep- 

 tionally great variation in the readings obtained from different 

 specimens of the same rock, as well as from the different sec- 

 tions in the same specimen. 



But such is not the case, as will be seen by an examination 

 of the figures in the accompanying table. These represent the 

 results obtained from ten measurements of the compressibility 

 of Baveno granite, which is coarse in grain, and ten of Sud- 

 bury diabase, which is very fine in grain, together with eight 

 measurements of Tennessee limestone, which is rather coarse 

 grain, and seven of 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. 



Maximum. Minimum. Difference. 

 Baveno granite (coarse), 10 



trials 4,880,000 4,380,000 500,000 



Sudbury diabase (very tine), 



10 trials 11,170,000 9,655,000 1,515,000 



Plate glass, 13 trials ... 6,930,000 6,020,000 910,000 



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 coarseness of grain and the magnitude of the variations in 

 the readings obtained. The differences in glass, which is an 



