344 ME. E. D. OLDHAM ON THE [&Ug. I907, 



22. The Constitution of the Inteeioe of the Eaeth, as eeyealed 

 by Eaethqtjaees : (Second Communication). Some New Light 

 on the Oeigin of the Oceans. By Bichaed Dixon Oldham, 

 E.G.S. (Bead June 19th, 1907.) 



The origin of the great oceanic basins has been the subject of 

 numerous speculations, more or less plausible, but none convincing : 

 for the reason that we have no knowledge as to whether these de- 

 pressions are mere surface-phenomena, or whether they are accom- 

 panied by, and related to, differences in the constitution of the 

 interior of the earth beneath them. It is known that the attraction 

 of gravity is, as a rule, greater at island- stations than on the con- 

 tinents, and that the plumb-line at coast-stations is frequently 

 deflected towards the sea; but these facts do not help, for they are 

 easily explicable by differences which may be regarded as superficial, 

 seeing that they need not extend to a depth of even as much as 

 one hundredth of the earth's radius. As regards greater depths, 

 mathematical deductions can help us but little, for any deductions 

 drawn in this way are subject to the logical objection that, while 

 showing the possibility of one explanation of known facts, they do 

 not exclude the possibility of some other. Direct or positive evidence 

 is therefore desirable, and this I propose to offer : not indeed 

 amounting to proof, but still strong enough to merit serious con- 

 sideration, that the ocean-basins of the Northern Atlantic and 

 Northern Pacific are not mere surface-departures from the perfect 

 spheroid, but are accompanied by differences in the constitution of 

 the matter underlying them, which extend to a considerable portion 

 of the whole radius of the earth. 



In a previous communication * I showed that the study of the 

 rate of transmission of earthquake-waves enabled us to reach some 

 positive conclusions regarding the constitution of the interior of 

 the earth. In that paper only the constitution of the earth as a 

 whole came into consideration, and only average values for all the 

 available earthquakes were dealt with ; this method is no longer 

 permissible when the detection of variations from the average is 

 aimed at, and it will be necessary to compare the rates of propa- 

 gation along different wave-paths with each other. The investi- 

 gation has been much simplified, and practically made possible, by 

 the fact that in 1906 there were two great earthquakes, both of 

 which were large enough to give very complete records at distant 

 stations ; both of which originated at about the same distance 

 from the group of seismological stations in "Western Europe, but 

 in such positions that the wave-paths differed radically in type. 

 Erom the San Erancisco earthquake of April 18th the wave-paths 

 ran under the continent of North America, crossed the Northern 

 Atlantic not far south of Iceland, and approached the obser- 

 vatories from the north-west. Almost the whole of the course of 



1 Quart. Journ. Geol. Soc. toL lxii (1906) pp. 456-73. 



