254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



importance in interpreting the geology of the earth's crust, is here 

 merely an incidental feature in the transition from granitic to ultra- 

 basic material. 



BASIC LATER 



Referring again to Figure 1, one may note that the earthquake 

 velocity curves run regularly and almost linearly from near the sur- 

 face to about 1,600 km. depth. It is natural to assume that this 

 region then is a more or less homogeneous material, the bulk modulus 

 and rigidity of which increase regularly with pressure. From 

 reasons given below it is probable that the normal density (i. e., 

 the density at low pressures) of this material is 3.3, which corre- 

 sponds to 3.35 at a depth of 60 km. The density at other depths may 

 be obtained by interpolation between the two curves of Figure 2. 

 Thus, at 1,600 km. depth the density has increased by compression 

 to 4.35. The normal density 3.3 corresponds to a pyroxenite or a 

 peridotite. Throughout this whole region the temperature must be 

 very high, and it is possible that a part of this layer is at a temper- 

 ature above its melting point, its high rigidity being maintained by 

 pressure. 29 Both the density and the earthquake velocity will prob- 

 ably be somewhat smaller in such a glassy material than in a crystal- 

 line layer of the same composition, but the difference can hardly be 

 great enough to nullify the evidence in favor of an ultrabasic layer. 



It has been suggested that meteorites should have the same average 

 composition as that of the earth or of any other part of the solar 

 system. Now this average composition 30 (due account being taken 

 of the proportion in which stony and metallic meteorites are seen to 

 fall) corresponds to: Olivine, 35; pyroxene, 42; anorthite, 4; 

 troilite, 5; nickel-iron, 13. The silicate portion is principally an 

 olivine-pyroxene mixture and thus is essentially a peridotite, and 

 should have nearly the same density and compressibility as that 

 postulated for the basic layer. 



PALLASITE LAYER 



As already noted, a remarkable feature of the earthquake velocity 

 curves (fig. 1) is the small amount of change beyond a depth of 

 1,600 km. From compressibility measurements the velocity of the 

 longitudinal waves in iron at moderate pressures is 6.1 km./sec, 

 whereas the velocity in peridotite is 7.2. At high pressures the 

 difference will probably be greater. This circumstance immediately 

 suggests that the nearly constant velocity below 1,600 km. maj r be 



20 Cf. R. A. Daly. Igneous Rocks and their Origin. (New York, 1914.) p. 172. Am. 

 Journ. Sci., 5, 349-371. 1923. 



80 Cf. O. C. Farrington. Field Columbian Museum, Geol. Ser... Vol. 3, Publ. No. 120: 

 211-13. 1911. W. D. Harkins. Journ. Am. Chem. Soc. 39 : 864. 1917. 



