James Geikie — On Changes of Climate. 217 



in tlie mud and sand of old sea-bottoms — land-surfaces have not 

 often been covered up and handed down. During the deposition 

 of the English glacial and interglacial drifts, Britain may have been 

 several times united to the continent and again separated, without 

 any record of such continental conditions having been preserved in 

 the marine drifts that border her maritime districts. For I hardly 

 suppose any geologist will seriously hold that the succession of the 

 drift deposits either in the east or north-west of England is con- 

 tinuous — that there are no gaps between the beds, but that from the 

 Forest-bed up to the latest postglacial deposit we have an unbroken 

 series, telling one connected tale. In Scotland the erosion during 

 glacial and interglacial periods was excessive ; and the superficial 

 deposits of England must also have been subjected to considerable 

 denudation throughout the whole cycle, from glacier ice, from pluvial 

 and river action, and from the sea. 



It may be said that the older river-gravels occupy valleys or 

 depressions which have been eroded through glacial drifts. But in 

 coming to this conclusion is it not just possible that geologists may 

 sometimes have been influenced by preconceived opinions with 

 regard to the age of the mammaliferous drifts ? It does not follow, 

 because a bed of Boulder-clay appears to be cut off by the slopes 

 of a valley, that the valley in question has been hollowed out since 

 the deposition of the Boulder-clay. To say the least, it seems just 

 as feasible a supposition that the valley, .with some portion of its 

 sand and gravel deposits, may have existed before the accumulation 

 of the Boulder-clay, and subsequently become partially filled up 

 with that bed, which, when the stream once more began to flow, 

 would be denuded and re-arranged or even swept away. During 

 this process the older alluvia or valley gravels would also be greatly 

 denuded; yet it is not at all improbable, but even highly likely, that 

 some portion would be spared. If this were the case, it would be 

 next to impossible to separate these out from the newer gravels 

 which oveidaid them, unless indeed the latter " should be found to 

 contain stones which could only have been derived from the Boulder- 

 clay. As an example of what is meant, reference may be made to 

 the highly interesting section of the Biddenham deposits given by 

 Mr. Wyatt.' The section is as follows : 



" Thin course of earth. 



Dark-red clay discolourations by infiltration, 



Subangular gravel, mostly worn, and chiefly composed of flints, small fragments 

 of iron shale of the greensand, and a few portions of the older rocks. 



Sand. 



Vein of sandy clay. 



Sand. 



Thin layer of black, apparently woody, matter. Selix, 



Fine gravel. 



Sandy clay layer. Succinea. 



Sand, occasional angular pieces of flint. 



Many shells, principally of the Succinea, Flanorbis, and Cyclas. 



Coarse gravel, boulders of red sandstone, flints, older rocks— some very large, 

 ochreous. 



' See The Geologist, 1861, p. 243; and Mr. Prestwich's paper in Quart. Journ. 

 Geol. Soc, vol. xvii., p. 364. 



