44 CORNISH POST-TERTIARY GEOLOGY. 



forest growth proves that elevation to have been far greater 

 than might be inferred from their present positions. The 

 stream-tin gravels, submerged forests, and overlying de- 

 posits, prove the long continuance of a subsidence bringing 

 about the present relations of sea and land. As, from its 

 nature, the Head is more likely to have been accumulated 

 during elevation, and for some time after the ensuing 

 subsidence commenced, I have no hesitation in regarding 

 it as older than the submerged forests. 



STREAM -TIN GRAVELS. 

 The stream-tin gravels, from their position, were evidently de- 

 posited prior to the growth of the forest stratum resting upon them, 

 and long before the submergence of the forests took place. The 

 depth of the tin ground below high- water mark (about 64 feet in 

 Mr. Henwood's section of Carnon) indicates a much more extended 

 coast-line. There are of course stream-tin gravels of all ages, but 

 those to which I refer are the deposits exposed in the valleys of Par, 

 Pentuan, and the Fal Estuary, etc., and all such inland gravels as 

 can safely be correlated with them. If these stanniferous gravels 

 were deposited prior to the Raised Beacli formation, we should expect 

 the following proofs : — 



(a) Deposits in the stream-tin valleys at heights corresponding to 



those of the neighbouring raised beaches. Whereas the 

 old estuarine gravel near the mouth of the Par Estuary, 

 corresponding in height to the raised beach near Spit 

 Point, rests on a slate platform in which the present bed 

 of the estuary has evidently been excavated. 



(b) To find traces of marine deposition or of a marine contour on 



the inland borders of flats like those of Ludgvan, between 

 Marazion and Penzance ; and these we do not find. 



(c) That relics of old fluviatile deposits would rest directly on 



the stream-tin gravels, whereas parts of the old forest- 

 ground rest upon them, and are overlain by fluviatile and 

 marine deposits, still more recent. 



(d) That the detritus of stream-tin gravels would be found 



amongst the materials composing the raised beaches, 

 which is not the case. 



