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X. — On the relative Ages of the Tertiary and Post-tertiary Deposits of the 



Basin of the Clyde. 



By JAMES SMITH, Esq., of Jordanhill, F.G.S. 



[Read November 6th, 1839.] 



Xn a former paper* I described the indications which I had observed of changes in 

 the level of the sea in the basin of the Clyde, which must have taken place at times, 

 geologically speaking, extremely recent, as the indications are met with above 

 the Till or erratic block deposit; and in a subsequent paperf I stated, that from 

 the Arctic character of several shells which have not been found recent in the 

 British Isles, that there was strong probable evidence that the climate was colder 

 at the period of their deposition than it is at present. 



Having given a detailed account of ray observations, with catalogues of the 

 recent and fossil shells of that district to the Wernerian Society (Wern. Mem. 

 vol. viii.), I confine myself, at present, to the results of observations made subse- 

 quently, which afford the most satisfactory evidence that in these comparatively 

 modern deposits, there are two distinct formations differing in climate and fauna, 

 and separated by a wide interval of time. 



If a difference in the testaceous fauna characterises the tertiary period, the oldest 

 of these, which contains about fifteen per cent, of extinct or unknown species, 

 must be held as a tertiary formation, and belongs to the newer Pliocene or Plei- 

 stocene of Lyell, and the newest, a post-tertiary one, as all the shells hitherto dis- 

 covered, agree with those of our present seas. Both of these deposits are anterior 

 to the recent or historical period. 



We have therefore, in the superficial or earthy covering of the older rocks, three 

 distinct epochs, the newer Pliocene, Post-tertiary, and Recent. To the fiirst of these 

 belong the till or diluvium and the stratified marine beds of sand, gravel, and clay 

 which lie over it, the shells of which contain a decided admixture of unknown 



* See Proceedings of the Geological Society, Nov. 16, 1836, vol. ii. p. 427. 

 t Ibid., April 24, 1839, vol. iii. p. 118. 

 VOL. VI. SECOND SERIES. X 



