Dr. A. Irving — The Thames High-level Plateau Gravels. 497 



to the changes of pressure. If my argument is valid, that for 

 solidity loses its force. ^ 



I will now give my reasons for thinking that the substratum, 

 if a liquid, is not a still liquid, but is affected by convection currents. 



Availing myself of Sir Arthur Eiicker's observed values of the 

 melting temperature and specific heat of Eowley rag, I have calculated 

 that, if the substratum of the crust be a still liquid, the thickness 

 of the crust comes out 22 miles, and the corresponding time since 

 it began to solidify about eight million years. This is a much 

 shorter time than geologists would admit. This result proves that 

 the substratum is not a still liquid, and must therefore be affected 

 by convection currents, bringing up heat from below and delaying 

 the thickening of the crust. The existence of convection currents 

 being thus, as I submit, established, I will add my reason for 

 believing that they ascend beneath the oceans. 



By a somewhat complicated calculation, which, although criticised 

 by Mr. Blake,^ has been ably defended by Mr. Brill," I have, I think, 

 proved that the substratum beneath the oceans is less dense than 

 beneath the land. This shows that the upward currents are beneath 

 the oceans. I have at the same time proved that the suboceanic 

 crust does not reach quite so deeply down as the continental crust, 

 and that its upper layer is thin and very dense, from which I infer 

 that it consists of basic lava-flows "^ the oxydation of which would 

 afford the red clay, which covers the bottom of the deeper oceans. 



These convection currents, ascending beneath the oceans and then 

 flowing horizontally towards and beneath the continents, till they 

 descend, are in my opinion the cause of the compression of the 

 continental crust. 



VIII. — The High-level Plateau Gravels on the North Side of 

 THE Tamisian Area, and their connexion with the Tertiary 

 History of Central England.^ 



By Alexander Trying, B.A., D.Sc. 



rpHE author refers to his work in former years among the High- 

 J_ level Plateau Gravels south of the Thames, chiefly in Berks 

 and Surrey, the results of which were given in various papers from 

 ten to twenty years ago.^ The present note may serve as a supple- 

 ment to those papers, in which the conclusion was arrived at that 

 the gravels in question were to be regarded as distinctly of riverine 

 origin and, upon the whole, of Pliocene age. Occupying original 



1 Proc. Camb. Phil. Soc, 1904. 



2 Phil. Mas:., 1894. 

 '■'■ Ibid., 1895. 



* " Physics of the Earth's Crust," Appendix, p. 8. 



5 A paper read before the British Association, Cambridge, Section C (Geology), 

 August, 19n4. 



'^ " The Bagshot Strata and their Associated Gravels," Proc. Geol. Assoc, vol. viii 

 (1883); "On the High Level or Plateau Gravels," Quart. Journ. Geol. Soc, 

 vol. xlvi (1890) ; Lecture at Windsor on "The Geological History of the Thames 

 Valley," Science Gossip, May and June, 1891 ; " On Svixface Changes in the London 

 Basin," Geol. Mag., May, 1893. 



