314 H. O. WOOD SOME CONSIDERATIONS TOUCHING ON ISOSTASY 



the earth at a depth of 30 to 50 kilometers is in a different condition 

 from that outside this depth. For example, on the basis of the length of 

 the seismic waves which are propagated over the surface, commonly about 

 63 kilometers, Wiechert deduced a depth of 31.5 kilometers for the "mag- 

 maschicht,'" which, as Galitzin 11 points out, agrees well with the depth of 

 the melting temperature on the basis of an increase of 1 degree centigrade 

 for each 36.7 meters of depth. Moreover, in one late study it has been 

 inferred that the matter immediately below a depth of about 50 kilo- 

 meters is in an expanded (and possibly molten) state, so that its density 

 is appreciably less than that of the immediately overlying material. This 

 study has not yet been published, so far as the writer is aware, but it is 

 based on observations in Samoa in connection with the arrival times and 

 characteristics, under known different conditions, of two differentiated 

 parts of the earliest or longitudinal waves registered by seismographs. 

 It is to be hoped that Angenheister will set this forth in the detailed and 

 critical form it deserves. 



It seems also that the prime motion in such displacements as those 

 detected by survey in connection with earthquakes, as in California in 

 1906, and in other instances, and the progressive northward shifting of 

 the International Latitude Observatory at Ukiah, California, recently 

 discerned and pointed out by Lawson, 12 finds its best explanation in the 

 drag of the overlying matter by a positive transfer of material in a sub- 

 jacent layer of ready yielding. 



These seismologic and similar findings, though as yet interpretations 

 of comparatively meager data, if sustained by further work will greatly 

 strengthen the geodetic argument for isostasy. 



Summary op Conclusions 



In conclusion, it is to be noted that vague terms, like yielding and 

 earth's outer sit ell, have been employed consistently in preference to more 

 specific or precise designations, such as plastic, viscous, fluid, etcetera, or 

 set, flow, shearing, recrystallization, etcetera, and crust or asthenosphere, 

 in order to avoid any misconceptions arising out of the definitions of these 

 terms, or their employment, in connection with more specific hypotheses. 



For the same reason no estimates of the depths of incipient or of ready 

 yielding have been given above. However, by implication the depth of 

 incipient yielding would be taken less than 30 kilometers on the basis of 

 the seismologic data, and of Willis's estimate of 40 miles as the depth of 

 the yield-point, taking no account of temperature increase downward. 



It has been attempted here to combine into a coherent whole true tend- 



