MODULUS OF ELASTICITY OF ROCKS. 



17 



TABLE VI. 



Though the variation of the elastic constant due to temperature- 

 rise is comparatively small, it can never be neglected in so far 

 as the velocity of seismic waves is concerned, since the under- 

 ground temperature rapidly rises with the increase of depth. 

 Although the elastic constants increase from Cainozoic ito Archaean 

 rocks in a greater ratio than the density does, to attain the main 

 stratum of Palaeozoic rocks we must go deep down some ten 

 kilometres, at least, and for a stratum of Archaean rocks, at least, 

 thirty kilometres, where the underground temperature must be 

 tolerably high. Any conclusive deductions should, however, be 

 postponed until the more accurate observations on the change of 

 the elastic constants due to temperature-rise, which are in course 

 of preparation, shall have fully elucidated the relation between 

 the elastic constants and the temperature. 



As a matter of fact, there are scattered everywhere within the 

 earth's crust veins and dikes of different kinds of old rocks, uplifted 

 by geological disturbances, some of which run over many hundreds 

 or thousands of kilometres. The velocity along such a vein or 

 dike must necessarily be greater than that through any of the 

 surrounding strata, so that the seismic waves mainly propagate 

 through that region. As a consequence of the above result, if 

 an observing station be situated near such a vein, not only will 

 the number of earthquakes observed at the station be greater 

 than those observed at any place in the vicinity, but the direction 

 of the motion will not necessarily indicate the position of the 



