410 



PROCEEDINGS. 



ETC. 



POSTPONED PAPERS. 



On the Age of the Tertiary Beds of the Tagus, ivith a Catalogue of 

 the Fossils. By James Smith, Esq. of Jorclan-liill, F.G.S. 



[Read June 30, 1S41.] 



[An abstract of this paper appeared in the Proceedings of the Geological Society, 

 Yol.iii.p.462.] 



During a recent yisit to Portugal my attention was chiefly directed 

 to an examination of the testaceous fossils of the tertiary deposits 

 near Lisbon, with the riew of ascertaining their geological age. The 

 result has been to satisfy me that they belong to the second great 

 division ui the ascending scale of the tertiary system, the miocene of 

 Lyell, and in particular that they agree more nearly with those of 

 the deposit of the south-west of France, in the emirons of Bordeaux 

 and Dax, than with any other which have been hitherto described"^'. 

 I was at first inchned to consider them to be, strictly speaking, identi- 

 cal and contemporaneous ; but a careful comparison of the Portuguese 

 fossils with those described in the catalogues of MM. De Basterct and 

 Grateloup, and also with a collection of shells from Bordeaux in my 

 own possession, shows a greater difference than can be ascribed to 

 geographical distance alone. But wiietlier they are more or less 

 ancient we have no means, in the present state of our knowiedge, of 

 determining, and the proportion of recent shells to extinct species 

 does not throw^ any light upon the subject. M. de Basterot makes 

 the proportion of recent shells in the Bordeaux fossils twenty-three 

 per cent., Avhilst M. Grateloup makes it thirty-seven per cent. ; accord- 

 ing to the amiexed catalogue, there are twenty-eight per cent, of recent 

 species in the Lisbon fossils, wiiich is somewhat below the mean, and 

 may perhaps indicate an earlier date. But although I am fully con- 

 vinced of the somidness of the prmciple by which we infer the age of 

 a tertiary deposit from the proportion of recent species which it con- 

 tains, yet in practice the difficulty of distinguishing species really dif- 

 ferent from mere varieties is so great, that in the present state of the 



* I have since found the same beds in the south of Spain and Malta. See 

 Proceedings of the Geological Society, vol. iii. p. 452, and Quarterly Journal of 

 the Geological Society, vol. iii. p. 52. 



