210 W. A. E. TJssher — Post- Tertiary Geology of Cornwall. 



Being chiefly composed of comminuted shells, the percolation of 

 water through the old dunes would best explain their consolidation. 

 Dr. Paris (T.K.G.S. Corn. vol. i. p. 7), in addition to this, gives two 

 other possible modes of consolidation, viz.. by water charged with 

 pyritical substances, or by ferruginous infiltration. 



The absence of organic remains in the majority of the Cornish 

 raised beaches has been ascribed to Arctic currents (God win- Austen, 

 Q.J.G.S. vol. vi. p. 87), which I think very probable. It also 

 suggests the possibility that many of them may have been deposited 

 by rivers, or in estuaries, whose seaward banks have been swept 

 away. Some of the boulder beds mentioned by Messrs. Carne and 

 Henwood are at too great heights to be regarded as raised beaches, 

 and may more reasonably be referred to far older fluviatile deposi- 

 tion. If the adit mentioned by Mr. Henwood (12 b) cut through 

 a continuation of the worn boulder beds of Porth Just and 

 Pornanvon to a thickness of 60 feet, the boulder beaches of these 

 localities must be regarded as anterior to the raised beaches. In the 

 formation of the old beach cliffs at the termination of a long period 

 of subsidence, fluviatile deposits (thrown down before, and during, 

 the initiation of the present lines of drainage) would have been 

 truncated, so to speak, and exposed at different heights upon the 

 cliffs, just as we find old river gravels exposed on the secondary 

 cliff-line of Devon. 



Again, during the elevation of the old beaches, the existing river 

 channels would have been deepened, and river deposits formed in 

 the breaches of the old cliff-line, to be redistributed by the sea in 

 its recent advance. How far boulder gravels and unfossiliferous 

 raised beaches (provisionally so called) may be referred to either of 

 these periods of fluviatile action it is impossible to say, without a 

 searching investigation of each particular deposit with references to 

 its surroundings. 



The local elevation of the raised beaches cannot be correctly 

 measured by the height above high water of their remains. For 

 such an estimate ignores the original thickness of the beaches, and 

 postulates an identity in the local rise of tide during the raised 

 beach formation and at present. The latter supposition is im- 

 probable when we take into account — 



lstly. The destruction by the sea during elevation (of the old 

 beaches) of such inequalities as may have proved obstacles to the 

 stream of tide. 



2ndly. The modification the raised sea-bed would have undergone 

 through subaerial agencies. 



3rdly. The probably different relations of land and sea in other 

 parts of England, and on neighbouring coasts during the formation 

 of raised beaches in the S.W. counties. 



4thly. The subsequent modification of the old coast-line. 



Mr. Pengelly (Trans. Dev. Assoc, part v. p. 103) points out the 

 fallacy of supposing — that all contemporary raised beaches are on 

 the same level, and the converse — that raised beaches on the same 

 level are necessarily contemporaneous. The cautions given show 



