part 2] TERTIARY OUTLIERS OF THE WEST OE ENGLAND. 207 



and their abundance in a rolled condition in the gravel at 

 Marazion, definitely indicated the post- Cretaceous age of each of 

 these deposits. In their general character, the gravels differ much 

 from the Pliocene strata of Cornwall and the Oligocene of Devon, 

 but they bear certain resemblances to some of the Eocene gravels 

 of the South-East of England. Their probable age was, therefore, 

 considered by Clement lleid to be Eocene. 1 



The plant-remains found in the lignites interbedcled with the 

 clays of the Bovey basin have established the age of the deposits as 

 Oligocene (Aquitanian). 3 They had been considered, however, to 

 be Middle Eocene, like the Bournemouth leaf-beds, by J. Starkie 

 Gardner, and were formerly referred to the Miocene when the 

 Aquitanian stage formed part of that formation in the classi- 

 fication of Continental authors. 



The lignitic clays and sands filling the Petrockstow basin are 

 similar in many respects to the Bovey deposits, 3 and are, therefore, 

 tentatively considered to be of the same age. 



Of the Pliocene deposits, those of St. Erth carry a fauna which 

 is considered by the majority of workers to date them as Pliocene, 

 the exact horizon being a matter of opinion. 4 The remanie blocks 

 of red sandstone around Lelant are similar in general characters to 

 the red sands at St. Erth. The sediments lying on the ' 400-foot 

 platform ' of Cornwall at Crouza Down and St. Agnes have been 

 referred to the Pliocene age, on account of their position and general 

 resemblance to some of the strata at St. Erth. The arguments for 

 the Pliocene or pre-Pliocene age of the 400-foot platform are too 

 well-known to need recapitulation here. 5 



All the samples examined were collected by me personally. 

 They were chosen, so far as might be, from every deposit exposed, 

 and selected to be as representative as possible, in regard both to 

 vertical succession and to lateral distribution. In order to save 

 space, the individual occurrences of minerals in each sample have 

 not been enumerated, but a summary table of occurrences for each 

 formation is given on p. 226. 



III. Petrography. 



(1) The Cretaceous Deposits. 



Earlier workers saw more complete sections of the Cretaceous 

 rocks than those visible on my various visits since 1912. The 

 lowermost beds recorded by H. J. Lowe, A. J. Jukes-Browne, 

 and Clement lleid have been obscured, and have therefore not 



1 Newton Abbot Memoir, p. 102. 2 Ibid. p. 106. 



:i W. E. A. Ussker, Q. J. G. S. vol. xxxiv (1878) p. 457 ; Trans. Devon. 

 Assoc, vol. xi (1879) p. 442. 



4 P. F. Kendall & E. G. Bell, Q. J. G. S. vol. xlii (1S86) p. 201 j A. Bell, 

 Trans. Roy. Geol. Soc. Cornwall, vol. xii (1898) p. 133; R. B. Newton, Journ. 

 Conch, vol. xv (1915) p. 56 ; and C. Reid, 'Pliocene Deposits of Britain' 

 Mem. Geol. Snrv. 1890. 



5 H. Dewey, Q. J. G. S. vol. lxxii (1916-17) p. 63. 



