ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXV 



M. Hebert has also given (p. 647) some further development to 

 his vievrs respecting that curious deposit of Rilly, with its large 

 Physa and numerous land and freshwater shells. This travertine with 

 the underlying white sands M. Hebert, in a previous memoir, had 

 placed at the base of all the tertiary beds, considering it a lake 

 deposit of which the greater part had been swept away prior to the 

 deposition of the marine sands of Rheims, which latter underlie in this 

 district the lignite and plastic clay series. In a paper communi- 

 cated to the Geological Society of France, Mr. Prestwich contested 

 these views, and endeavoured to show that the white sands of Rilly 

 were merely an altered and local condition of the marine sands of 

 Rheims, that the Rilly travertine was consequently newer than these, 

 and that it belonged to the base of the lignite and plastic clay series. 

 As the group of organic remains belonging to these beds is one per 

 se, the solution of this question depends upon stratigraphical evi- 

 dence. M. Hebert now adduces further arguments in support of his 

 opinion, and shows that this travertine has a far wider range than 

 before known, having found it at Dormans, and extending to the 

 confines of the department of the Oise. 



M. Ami Boue gives (Bulletin, vol. xi. p. 61) the result of some 

 curious researches on which he has been engaged respecting the 

 depth of former seas and the height of former mountains, and con- 

 cludes that, taking the mean depth of the present seas to be from 

 12,000 to 18,000 metres, there is a gradual decrease from that 

 depth to the seas of the older geological periods — the Tertiary seas 

 having averaged 10,000 to 16,000 metres, the Cretaceous 8000 

 metres, and finally the Permian and older seas 3500 to 9000 metres. 

 At the same time he estimates that the mountain chains of the earth 

 have now attained their greatest elevation, or a mean which he 

 makes exactly equal to the depth of the present seas given above : 

 in the same proportion he fixes the height of the older moun- 

 tains, making those of the Permian and Silurian period 3500 up to 

 9000 metres in heia;ht. He also estimates the mean elevation of 

 the present general surface of the land (mountains excepted) to be 

 300 metres, gradually decreasing to 100 and even 60 metres during 

 the Permian and older periods. 



My predecessor in this chair alluded in his address at our last 

 anniversary to the " Treatise on the Tertiaries of the Mayence Basin," 

 by Dr. F. Sandberger of Wiesbaden, but delayed entering on the 

 consideration of its contents, as well as of those of the works of other 

 German writers, especially of Beyrich and Diinker, until the paper 

 by myself on the same Mayence tertiaries, and which was then 

 announced, should have been read before the Society. This paper 

 having now been read, as well as another paper on the Tertiaries of 

 Hesse Cassel, I shall endeavour to fill up the hiatus left by our late 

 lamented President by putting together, as well as I can, the infor- 

 mation already obtained respecting the Tertiary formations of Ger- 

 many. Our late President would have done full justice to this 

 interesting subject ; I can only attempt to follow his footsteps at a 

 distance. 



