64 Prof. H. Nagaoka on the Elastic Constants of 



earth's crust. The chlorite schist of." Chichibu has a density 

 nearly equal to 3, although its modulus of elasticity is greater 

 than that of brass or copper with a rod cut in the direction of 

 strongest tenacity, it is so brittle in the direction perpendicular 

 to it that it is impossible to obtain a single specimen with 

 which the elastic constant can be accurately determined. The 

 elastic constants are widely different as the specimen is cut 

 in one or other direction, especially in archsean and palaeozoic 

 rocks, as schists and slates with distinct sedimentation planes. 

 Rocks of eruptive origin are generally free from such direc- 

 tional behaviour; but when they are pressed or otherwise 

 subject to continuous application of stress, the difference of 

 elasticity in different directions can still be traced. Such 

 appears to be the case with marble and granite. 



The elastic constants of archaean and palaeozoic rocks are far 

 superior to those of the cainozoic; but the velocity of propa- 

 gation of longitudinal or transversal waves is not proportionally 

 large. As the ratio of the elastic constant to density deter- 

 mines the velocity of propagation, we cannot at once conclude 

 from the increase of elasticity that the waves travel with 

 greater velocity. It would be too bold to draw anything like 

 a general conclusion from the examination of some eighty 

 specimens ; but so far as the present experiments go, the 

 tendency is such that the elastic constants increase more 

 rapidly than the density as the rock becomes denser, and 

 consequently elastic waves travel with greater velocity in the 

 interior than on the surface of the earth's crust. Eruptive 

 rocks are more isotropic than those of non-igneous origin, 

 and have inferior elasticity; but there is the same distinction 

 with age. Elastic waves in eruptive palaeozoic rocks travel 

 with slower velocity than in those of the archaean of the same 

 origin ; a similar remark applies to cainozoic rocks with a 

 few exceptions. 



As we go deep in the earth's crust the rocks generally 

 assume schistose structure; we have reason to believe that the 

 elastic constants of the constituent rocks increase in a certain 

 particular direction, which evidently coincides with that of 

 swiftest propagation of elastic disturbance. Pressed by the 

 weight of the superincumbent crust, these rocks will be of 

 greater density; so that the increase of elastic constants is 

 attended with corresponding increase of density. We cannot 

 conceive that the elastic constant or the density will conti- 

 nually increase as we approach the centre of the earth ; they 

 will both attain asymptotic values. The alternatives are : 

 either the ratio of elastic constants to density goes on gra- 

 dually increasing, or it first reaches a maximum and then 



