358 
Superficial Gravels and Clays. 
[July, 
lake it is impossible to say. We have evidence, however, 
that it was long enough for some of the land-shells to 
occupy the surface again, and also, as the buried forest-bed 
at Ealing witnesses, for small trees to grow up. Until very 
recently I had not obtained this evidence, and was under 
the impression that the lake had been quickly re-formed. 
The forest-bed at Ealing shows clearly, however, that some 
years at least must have elapsed before this took place. I 
have found land-shells at Brentford, and near Bletchley, in 
Buckinghamshire, at this horizon. 
The Rev. H. M. De la Condamine described, in 1853, a 
deposit containing land and fresh-water shells on the top of 
the Middle Sands and Gravels, in the valley of the Ouse, 
between St. Ives and Huntingdon.* At Crayford there is a 
grey sandy clay containing shells of the fresh-water mol- 
lusks, Planorbis, Bythinia , Lymnea, and Anodon. The Anodon 
has the two valves united. The clay with these shells over- 
lies the mammaliferous gravels and underlies the deposits 
that appear to be the representatives of the Upper Boulder- 
clay, so that I think it must belong to this stage. It was 
probably at this time that the land shells contained in the 
Upper Loess and Diluvium multiplied greatly, and occupied 
the land, many of their natural enemies and competitors 
having been destroyed by the flood. 
It seems not impossible that remnants of palaeolithic 
tribes or some of the great extinCt mammals might have 
escaped destruction at the time of the rising of the waters 
of the first lake, and have spread over the area again before 
it was again submerged. I have, however, so far found no 
evidence whatever that they did so. The scarcity, if not 
absolute non-existence, of the bones of the species of deer, 
horse, and ox, that we know were not exterminated, makes 
it probable that the gap in the ice-barrier did not remain 
many years open. 
A. Upper Diluvium. — The Upper Diluvium, under which 
name I include the Upper Boulder-clay, the Upper Loess, 
the Hessle Clay, and the Upper Brick-clays, is the most 
widespread and the best preserved of all the glacial deposits. 
The gap in the ice-barrier had been closed and the lake 
re-formed, but this time the water was not suddenly dis- 
charged, and gradually cut out a channel between the Black 
Sea and the Mediterranean. There are several faCts 
pointing to the probability that it was the channel of the 
* Quart. Journ. Geol. Soc., vol. ix., p. 271, 
