186 Reports and Proceedinrjs — Geological Society of London. 



F.R.S. ; C. Fox Strangways ; J. J. TI. Teall, M.A., D.Sc, F.R.S. ; R. H. 

 Tiddeman, Jf.A. ; Professor" W. W. "Watte, M.A., M.Sc, F.R.S.; Rev. H. H. 

 "Winwood, M.A. ; A. S. Woodward, LL.D., F.R.S., F.L.S. ; and H. B. 

 Woodward, F.R.S. 



Officers : — President : Sir Archibald Geikie, Sc.D., D.C.L., LL.D., Sec.R.S. 

 Vice-Presidents: R. S. Herries, M.A. ; J. E. Marr, Sc.D., F.R.S. ; A. Strahan, 

 M.A., F.R.S. ; and J. J. H. Teall, M.A., D.Sc, F.R.S. Secretaries: Professor 

 E. J. Garwood, M.A., and Professor W. W. Watts, M.A., M.Sc, F.R.S. 

 Foreign Secretary: Sir John Evans, K.C.B., D.C.L., LL.D., F.R.S., F.L.S. 

 Treasurer: H. W. Monckton, Treas.L.S. 



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

 Sec. R.S., President, in the Chair. The following communi- 

 cations were read : — 



1. " The Constitution of the Interior of the Earth, as revealed by 

 Earthquakes." By Eichard 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 tliird represents wave - motion which has 

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

 papex', 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 fi'om 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 inci'eased 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 trans- 

 mission 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. Tlie 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 orieriii must have reached their 



