398 Jones — Review of Chamberlin's Groundwork 



Now temperature increase is so very much more effec- 

 tive in decreasing the density of material than pressure 

 increase is in increasing its density that it is unsafe to 

 assume that materials within the earth are in their most 

 dense or crystalline state, much less to assume that some 

 unknown compressional state exists. Again, so far as 

 high pressure experiments have extended, the density 

 increase upon pressure increase becomes negligible. The 

 rigidity, however, progressively increases. The increase 

 of density in the depths of the earth, it would seem, must 

 be independent of high pressure and so independent of 

 rigidity. 



The Evidence of Seismic Vibrations. — Since records 

 of the transmission of seismic vibrations constitute the 

 only direct evidence we have of the state or states of the 

 earth's interior an analysis of this evidence becomes 

 imperative. Seismic disturbances send out vibrations of 

 two types, compressional and distortional. These are 

 called the primary and secondary waves respectively. 

 The former waves are dependent on the elasticity or 

 compressibility of the transmitting medium, while the 

 latter waves are dependent both on the rigidity and the 

 elasticity of the transmitting medium, for their propa- 

 gation. The two types of waves travel at different veloci- 

 ties but can only become distinctly separated out in a 

 homogeneous medium ; that is, homogeneous as to stress 

 effects or, in other words, isotropic. The resultant vibra- 

 tions which travel circumf erentially from the shock center 

 pass through what we know is a heterogeneous medium. 

 The wave types are not separated in the earth's surficial 

 shell. 



At a minimum distance of 700 miles, or 10 degrees of 

 arc, from the epicenter a three-phase record becomes 

 decipherable. The first phase vibrations to be recorded 

 are the compressional or primary waves ; next come the 

 second phase distortional or secondary waves, and both 

 these phases come through the earth. Finally the third 

 phase, undifferentiated waves, reaches the recording 

 device via a circumferential path. 



From the mere fact that the primary and secondary 

 waves are separated at distances above 10° of arc from 

 the epicenter "it is clear," according to Davison, 10 "that 



18 C. Davison: A Manual of Seismology, Cambridge, 1921, p. 149. 



