CORNISH POST-TERTIARY GEOLOGY. 45 



Hence I conclude that the stanniferous gravels are more modern 

 than the raised beaches, and not only posterior to them, but sepa- 

 rated by a long lapse of time, during which denuding agencies were 

 ceaselessly at work, and great changes took place in the physical 

 geograph}'^ of Cornwall. 



SUBMERGED FORESTS. 



The growth of the old forest, the relics of which have been met 

 with all round the Cornish coast, must have extended over a long 

 period of time. The evident connexion of the Mounts Bay Forest 

 with the bed in Marazion Marsh overlying stream-tin, pointed out 

 by Mr. Carne ; and the constant presence of a distinct vegetable 

 stratum, or of detritus mixed with vegetable matter, on the tin 

 gravels in most of the principal sections, points to a general corre- 

 lation of the submerged forests on the coasts with the forest bed in 

 stream-tin sections. Although the forests may have flourished during 

 the deposition of the stanniferous gravels, for purposes of classifica- 

 tion it is more convenient to regard them as bridging over the 

 interval between the deposition of the principal stanniferous gravels 

 and the much more modern fluviatile, marine, and estuarine deposits, 

 beneath which they were entombed. 



Several observers have cited the occurrence of hazel nuts, in some 

 cases with branches attached, as proof of a sudden autumnal sub- 

 mergence. As the forests flourished on a slightly modified marine 

 contour, which would probably exhibit a series of terraciform features, 

 during a gradual subsidence sudden encroachments of the sea would 

 take place as these plateaux were successively submerged. 



The peaty matter so constantly associated with the forest bed, 

 though it might in some cases be explained by the saturation of an 

 old vegetable soil forming round the trees for centuries, would, as a 

 rule, impress one with the idea that the forest tracts were converted 

 into marshes, by the formation of gravel or sand bars damming back 

 the drainage of the valleys, for some time before the sea regained its 

 old cliff bounds. 



The general absence of trees in the central and western parts of 

 Cornwall, owing to the prejudicial influences of proximity to the sea, 

 leads me to think that the forests began to flourish, either during 



