ON THE TRANSIT-VELOCITY OP EARTHQUAKE WAVES. 



229 



An examination of these Tables presents some remarkable and, so far as 

 I am aware, now for the first time observed results. 



As might have been expected, the quartz-rock is much less compressible 

 generally than the slate-rock, with this exception, however, that the softest 

 specimens of quartz-rock, and those alone, are much more compressible than 

 the softest slate, when both compressed in the direction of or parallel to the 

 lamination. 



i In this direction of compression, the hardest slate is more than double as 

 compressible as the hardest quartz. 



When compressed transverse to the lamina, however, the hard slate and 

 hard quartz prove to have very nearly the same coefficient of compres- 

 sibility, which is very small for both ; while the softest slate and the softest 

 quartz, compressed in the same way (transverse to lamina), have also nearly 

 the same coefficient of compressibility, but one about four times as great as 

 for the hardest like rocks. 



These facts point towards the circumstance of the original deposit and 

 formation of these rocks as their efficient causes. Both rocks consist of 



