40 CORNISH POST-TERTIARY GEOLOGY. 



BOULDER GRAVELS. 



As in the granitic districts of the Land's End, the modern beacbes 

 are often almost entirely composed of large rounded granite boulders, 

 it is not improbable that many of the boulder gravels alluded to by 

 Messrs. Came and Henwood ; at Pornanvon, St. Loy, and St. Just, 

 for instance; may represent raised beaches or contemporaneous 

 fluviatile deposits, whilst others in valley bottoms of the present 

 lines of drainage may be equivalent to stanniferous gravels else- 

 where. But boulder gravels, such as those mentioned by Mr. 

 Henwood as occurring at a height of 80 feet above the sea at 

 Morrab Place, Penzance, or even the boulder-bed forming the roof 

 of Gamper Hole, at 40 feet above the sea, mentioned by Mr. Came, 

 cannot be classified with the Raised Beaches, even on the supposition 

 of unequal elevation, as well as a local rise of tide : and still less 

 can they be regarded as equivalent to stanniferous gravels. 



It is vei"y difficult to believe that no traces of old fluviatile depo- 

 sition, prior to the formation of the old beach cliffs (i.e. before and 

 during the subsidence which culminated in the formation of the 

 Raised Beaches), were preserved. Before and during the elaboration 

 of the present drainage system, deposits would no doubt have been 

 formed marking the progress of denudation by the rivers. Whilst 

 in inland districts the great surface waste during the period of the 

 accumulation of the Head on the coasts would have tended to the 

 dispersion and concealment of such old fluviatile relics on the slopes, 

 mining excavations, and the wearing back of the coast-line would 

 naturally bring them to light. The boulders at Huel Carn Mine, 

 50 feet from the surface and 500 feet above the sea, mentioned by 

 Mr. Carne as occurring beneath a mass of disintegrated granite and 

 clay, are probably explainable in this way. The boulders of granite 

 in Rosewarne Mine, Gwinear, at 74 fathoms from the surface, noticed 

 by Mr. Salmon who considered that they had been introduced by 

 fissures from the surface, are so difiicult of explanation without 

 further particulars, especially the description of the structure of the 

 granite in their vicinity, that I hesitate to hazard a conjecture re- 

 specting them. At Relistian Mine, on the contrary, though the 

 slate pebbles observed by Mr. Carne were met with at 100 fathoms 

 from the surface, his mention of spheroidal concretions in the slate 

 in their vicinity seems to offer at once the simplest and most reason- 



