MT. 66.] GEOLOGY AND MATHEMATICS. 275 



Tulle and Brives, and then a short visit to some pits in the 

 valleys of the Vezere and Dordogne. 



Then Toulouse, St Gaudens, Tarbes, Pau, Bayonne, Biarritz, St 

 Jean de Luz, and Irun for caves and coast-sections. 



When planned, I must write to L. Lartet and Eaulin. I hope 

 to see you on Wednesday. . . . 



You see I omit Limoges and keep farther east. My wife says 

 I am not to be allowed to go unless you are of the party. She 

 has just been looking at the map, and condemns somewhat 

 strongly the Central France portion hills, mountains, precipices, 

 frost, and snow ; but approves of the South of France part. I 

 tell her we can report of the country when we come back. 



J. Prestwicli to Rev. 0. Fisher. OXFORD, llth March 1878. 



MY DEAR FISHER, You encourage me to ask you any geo- 

 logical question involving mathematical investigation. There 

 is one I have been considering, and which you have already 

 closely touched upon. Is it possible that earthquake waves 

 can be transmitted by a crust so rigid as that which Sir W. 

 Thomson l would establish ? In those cases where trees wave 

 from side to side, and we have other evidences of a rolling 

 motion of the crust, it seems to me impossible that a mere 

 vibration of the shock is sufficient. Such vibration may be 

 transmitted, but it seems to me hardly in that form and to 

 such distances. 



What think you on mathematical grounds ? Are you going 

 to pay Oxford a visit this term? I have yet two weeks of 

 lectures on Tertiary and Quaternary Strata. Next term I com- 

 mence excursions, and lecture on the ground to be visited, If 

 you can join in any of them it will give me much pleasure. 

 Mrs Prestwich desires her kind regards, and hoping you and 

 your boys are all well, I am, sincerely yours, 



Jos. PRESTWICH. 



J. Prestivich to Rev. 0. Fisher. OXFORD, 28th March [1878], 



MY DEAR FISHER, Thanks for your note and explanations. 

 I can quite understand the propagation of a wave in a given 



1 Now Lord Kelvin. 



