Mocks and the Velocity of Seismic Waves. 59 



It will be seen that the initial deflexion amounts to 47'8 mm.; 

 the torsion of the prism gradually increases in course of a few 

 minutes, so that after a lapse of about 19 minutes the increase 

 of deflexion is nearly 30 per cent, of the initial. The increase 

 becomes asymptotic with time. 



The above-mentioned property of rocks will be of no small 

 interest in dynamical geology, as it naturally illustrates the 

 possibility of the folding of rocks and other kindred phe- 

 nomena pertaining to the manifold change of shape in rocks, 

 wrought by the continuous action of stress. 



Velocity of Elastic Waves. — It was my intention to deter- 

 mine the modulus of elasticity, and then calculate the velocity 

 of propagation of the longitudinal as well as that of the 

 transversal waves, on the supposition that the material is 

 isotropic. A few experiments with rocks of different ages 

 showed that these attempts are for the most part fruitless, as 

 the assumption of isotropy was not generally admissible. With 

 archsean and palaeozoic rocks it was possible to work them into 

 proper shape for experiment only in a certain direction, as 

 they were generally of schistose structure, and extremely 

 brittle in the direction perpendicular to it ; in such cases the 

 elastic behaviour was of course widely different in these 

 directions. Even with granite, which apparently is homo- 

 geneous in structure, the difference of elasticity with direction 

 was noticed. On examination, these rocks were found to have 

 been pressed from one side during formation, and this left its 

 trace in the relation of strain to stress. For the complete 

 discussion of the elastic nature of these rocks the determination 

 of the moduli of elasticity and of rigidity considered as an 

 isotropic substance is insufficient; we are in fact dealing with 

 quasi-crystalline bodies, so that the number of elastic constants 

 must depend on the number of symmetry-planes which can be 

 drawn in these rocks. The type of the elastic waves travelling 

 in such a medium will be determined, when all of these 

 constants are known. As we have no simple means of 

 examining these symmetry-planes, a single modulus of elas- 

 ticity and rigidity was determined, on the supposition that 

 the material is isotropic. 



In the discussion of the propagation of seismic waves we 

 have to deal with wave-lengths which measure over a kilo- 

 metre. Geologists tell us that uniform strata of a kilometre 

 thickness are of rare occurrence; and it may be doubted if 

 these waves do not suffer change of type and shape in tra- 

 versing the earth's crust. Unquestionably longitudinal plane 

 waves whose velocity of propagation in an isotropic medium 



