i8yS.} 
Superficial Gravels and Clays. 
357 
the violence of the western flood. It is in this sheltered 
bay that the mammalian remains, so abundant between East 
Adton and Brentford, have been preserved. Mr. Alfred Tylor, 
some years ago, drew attention to the fa<5t that all the 
mammaliferous deposits of the Thames Valley occur in 
situations similarly protected by escarpments to the west- 
ward ; and my experience has been to the same effedt. 
We have thus, on the one hand, deposits completely 
broken up by the violence of the flood, and, on the other, 
completely preserved from it. All the gradations between 
these two extremes should and do exist. In some the re- 
mains have been broken and mixed through the gravel, and 
even sorted according to their specific gravity. In others 
they have been drifted for a short distance, and the two 
mammalian faunas are mixed together without the bones 
being broken or much rolled. In others they have been only 
a little drifted from their original position ; and in others, 
again, remain as they were first deposited, the bones of the 
same animal lying together. 
Much confusion must thus arise, and much difficulty in 
determining the succession of the beds. The difficulty is 
often increased when the remains lie at the junction of 
tributaries with the main stream. In such cases it has 
sometimes happened that the water filling the second- 
ary valleys has been pounded back by the greater flood 
coming down the main one, so that, after the gravels 
have been spread out by the latter, the contents of the 
former have been washed over them. And thus, pre-diluvial 
mammaliferous sands lying in the tributary valleys, and 
protected from the violence of the flood coming down the 
principal channel, have, by the change in the direction of 
the currents during the falling of the water-level, become 
interstratffied with — or even shifted unto the top of — much 
newer deposits. The only way to guard against being de- 
ceived by instances of this kind is not to accept any evidence 
of superposition as conclusive where the bones of the same 
animals do not lie near together, where the lamellibranchiate 
shells have not their two valves united, or where there are 
other signs of the deposit being a drifted one. 
In Norfolk and Suffolk the Middle Sands and Gravels 
often contain fragments of marine shells derived from the 
Weybourne Sands and other beds. In Yorkshire they are 
known as the Hessle Sands and Gravels. 
0 
B. Land Surface . — How long the gap in the ice-barrier 
remained open after the discharge of the water of the first 
