5o8 JOSEPH BARRELL 



surface to 12.5 km. per second at 2,170 km. from the surface, 0.66 

 of the radius from the center. Below that depth the velocity is 

 nearly constant. The velocity of the transverse waves is 4.8 km. 

 per second at the surface and increases almost linearly with depth, 

 reaching a velocity of about 7 . 5 km. per second at half the distance 

 to the center of the earth. The absence of good records from 

 distances beyond 125° prevents a knowledge of the velocities at 

 greater depths. Within the limits regarding which information is 

 given, Reid remarks that there is no indication of a sudden change 

 in the velocity of either wave such as we should expect if there were 

 any sudden changes in the nature of the earth's interior. Oldham 

 also finds no evidence of. sudden change to a depth of at least 

 2,400 miles, 0.4 radius from the center.' From the curves showing 

 the relation of velocity to depth which Reid gives^ it is seen that 

 the ratio of velocity of the transverse to the velocity of the longi- 

 tudinal wave is o . 66 at the surface, 0.56 at o . 95 R, o . 53 at o . 9 R, 

 reaching a minimum of 0.52 at 0.85 R, from which it increases to 

 o. 58 at o. 5 R. This shows that both moduli of elasticity increase 

 with depth, but that down to a depth of between 0.8 and 0.9 R. 

 from the center of the earth, 637 and 1,274 km. from the surface, in- 

 compressibility increases relatively faster than rigidity. The change 

 is shown as very rapid in the first 300 km. This is the only way 

 in which the existence of an asthenosphere reflects itself in the 

 rigidity of the earth, and this may not be related to its weakness 

 but to some other property, such as the nature of compressibility 

 or of changing chemical composition, or partly in the lack of 

 detailed knowledge in the nature of the data. 



Earthquake waves, Uke the tides, measure elasticity rather 

 than strength. The vibrations which penetrate 200-300 km., 

 and more, downward in the earth are already greatly reduced in 

 amplitude and therefore in the strains which they bring on the 

 earth. What the maximum strains may be is unknown, but reason- 

 able assumptions as to amplitude show that within the astheno- 

 sphere the order of magnitude of the strains would be of the nature 



^ "On the Constitution of the Interior of the Earth as Revealed by Earthquakes," 

 Quar. Jour. Geol. Soc, LXII (1906), p. 470. 

 ^ P. 122. 



