EARTH *S INTERIOR ADAMS AND WILLIAMSON 253 



must increase with depth, although the increase is not neces- 

 sarily regular. Probably the outer 10 to 20 km. has the average com- 

 position of a granite or a granodiorite. From the seismographic rec- 

 ords of the Oppau explosion, Wrinch and Jeffreys 25 find the velocity 

 of the longitudinal waves to be 5.4 km./sec, and Mohorovicic 26 by 

 analysis of the records of near earthquakes concludes that the velocity 

 from the surface to a depth of 60 km. is nearly constant and equal to 

 5.8 km./sec. The data given in Table 2 enable us to use these observa- 

 tions to determine the composition of the outermost layer and to com- 

 pare it with the thin surface film accessible to direct examination. 

 Theoretically the surface velocity can be obtained from the initial 

 slope of the ordinary time-distance curve, but on account of the 

 scarcity of reliable observations for near earthquakes the extrapola- 

 tion of the surface velocity back to zero distance is unsatisfactory, and, 

 moreover, as emphasized by Wrinch and Jeffreys, the usual uncer- 

 tainty regarding the depth of focus would vitiate the results at short 

 distances. From Turner's table the " surface " velocity of the longi- 

 tudinal waves seems to be about 7.1 km./sec. — between the values for 

 pyroxenite (7) and for peridotite (7.2), and distinctly higher than 

 that for gabbro (6.9). According to Mohorovicic, the velocity of 

 the longitudinal waves suddenly changes from 5.8 to 7.9 km./sec. 

 at a depth of 60 km. From Table 2 it may be seen that the velocity 

 in granite is 5.6 km./sec. The seismologic data, therefore, although 

 none too satisfactory, seem clearly to indicate that the crust of the 

 earth is largely granitic in character, but that at a depth of 100 km. 

 or less, basic material becomes predominant. 



We propose, somewhat arbitrarily, to take 60 km. for the thick- 

 ness of the layer in which the rocks change from acid to basic. 

 Whether or not the change is gradual is a question to be decided 

 at some future time. The lower limit of this layer may or may not 

 be identical with the depth of isostatic compensation. From gravity 

 measurements in mountainous regions this depth is placed by 

 Bowie 27 at 96 km., but from the data over the whole United States 

 he places it at 60 km. Washington, 28 moreover, finds the average 

 density of various regions on the earth to harmonize with the aver- 

 age elevation on the basis of isostatic compensation at a depth of 

 59 km. In any case this layer has a volume of only a few per cent 

 of the total volume of the earth, and its thickness has little effect 

 on the density distribution of the earth as a whole. The basaltic 

 substratum, postulated by Daly, Wegener, and others, and of great 



25 Dorothy Wrinch and Harold Jeffreys. Roy. Astr. Soc, M. N., Geophys. Suppl. 1, pp. 

 15-22. 1923. 

 28 Lac. oit. 



"W. Bowie. U. S. Coast and Geod. Survey, Sp. Publ. No. 40, 133. 1917. 

 28 H. S. Washington. Op. cit., p. 405. 



1454—25 18 



