﻿[ 165 ] 

 XXV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 96.] 



February 21st, 1906.— Sir Archibald Geikie, D.C.L., Sc.D., 

 Sec.R.S., President, in the Chair. 



^HE following communications were read : — 



-*■ 1. 'The Constitution of the Interior of the Earth, as revealed by 



Earthquakes.' By Richard Dixon Oldham, F.G.S. 



This paper is not devoted to a fresh theory of the earth, but is 

 intended to set forth some of the information which can be obtained 

 from the study of the records of distant earthquakes. The modern 

 seismograph has given to geology a new instrument of research, and 

 extended its scope much in the same way as the spectroscope 

 extended the scope of astronomy, by enabling us to see into, and 

 determine the physical constitution of, the interior of the earth at 

 depths removed from any other possible means of research. 



The distant record of a great earthquake exhibits three distinct 

 phases, of which the third represents wave-motion which has 

 travelled along the surface of the earth and is not dealt with in this 

 paper, as it can give no information regarding the interior of the 

 earth. The other two phases form the preliminary tremors, and it 

 is shown that they represent the emergence of two distinct forms of 

 wave-motion, which have been propagated through the earth. 



A study of the intervals taken by these waves to reach remote 

 points shows that, up to a distance of 120° of arc from the origin, 

 they are propagated at a rate which increases with the depth of the 

 wave-path, and reaches an average of over 10 km. sec. in the case 

 of the first-phase, and over 6 km. sec. in the case of the second- 

 phase waves. The increase, being gradual and continuous, may be 

 attributed to the effect of increased pressure and temperature, and 

 there is no indication of any change in physical constitution of the 

 material traversed by waves which emerge at 120° or less from the 

 origin. 



Beyond this limit the first-phase waves show a reduction in the 

 mean rate of transmission, while the second-phase waves are 

 not to be found where they would be expected, but at about 11' 

 later. The interpretation is that the wave-paths emerging at these 

 greater distances have entered a central core, in which the rate of 

 transmission of the first-phase waves is reduced to about nine- 

 tenths, and of the second-phase waves to about one half, of the rate 

 in the outer shell. The great reduction of rate in the case of the 

 second-phase waves means great refraction, and the wave-paths 

 which emerge at distances of over 130° from the origin must have 



