CORNISH POST-TERTIARY GEOLOGY. 41 



able explanation. Numerous localities in the Land's End District, 

 in which boulders occur, have been cited by Mr. Carne in Trans. 

 Koy. Geol. Soc. Corn. vol. iii. It is to be regretted that the absence 

 of similar notices of the old gravels in the central and eastern part 

 of Cornwall renders this part of the subject so defective. 



When we reconstruct, in imagination, the Cornwall of early 

 Pleistocene times, before the submergence which led to the Raised 

 Beach formation had begun, we can scarcely consider the present 

 extent of the county as embracing more than the main watershed 

 boundary and sources of the old drainage system, so that the paucity 

 of old river gravels is not to be wondered at. 



EAISED BEACHES. 



Erom the sections given, the average height of the base of the 

 Eaised Beaches may be taken as from 5 to 10 feet above high-water 

 mark ; but, as a considerable thickness of beach has been denuded 

 away from the sites, where traces are only now visible, and as in 

 many places the Head, or talus, was shed upon it while in an uncon- 

 solidated state, partly sweeping away and partly becoming mixed 

 with its sands and pebbles, sections where the whole thickness of 

 beach seems to be present (as in Gerran's Bay, St. Ives, Godrevy 

 Beach, and Fistral Bay) must be taken, which would raise the 

 average extent of subsidence during the Raised Beach formation to 

 15 feet above high- water mark. 



As, no doubt, beaches were in some cases heaped up to a few feet 

 above S2:)ring-tide high-water, or were, like the modern West Green 

 Sandbank near Marazion, overspread by the shifting sand grains 

 drifted from the foreshore, the actual thickness of Eaised Beaches, 

 where undenuded, is not always a safe index of the amount of sub- 

 sidence. 



The height of the old beaches would naturally be greater in 

 proximity to their cliff-line, so that isolated portions, on reefs at the 

 level of spring-tide high-water, or exposed at the base of a consider- 

 able thickness of Head, masking an old cliff some distance inland, 

 are no indices of the local amount of elevation of the beaches.' 



1 Vide De la Beche, Geological Manual, p. 157 ; and Pengelly, " On Raised 

 Beaches," Trans. Dev. Assoc, for 1866. 



