60 PEOCEEDIXGS OP THE GEOLOGICAL SOCIETr. 



districts in the gravel-period as there is now. I have intro- 

 dnced ilkistrations of these "Welsh gravels, because it would be im- 

 possible to form any clear view of the general features of the gravel- 

 period without comparing the sections of the deposits found on the 

 upper part of rivers flowing off high land with those formed along 

 the lower parts of rivers near the sea in the Quaternary period. The 

 general law of deposition will be the same ; and it is this law I wish 

 to investigate by the aid of the measured sections. Those sections are 

 chosen specially for description where the rock or clay and the in- 

 cumbent gravel can be observed together. This gives an opportunity 

 of observing the points or lines on which denudation ceases and de- 

 position commences. 



In the London basin, the series of Quaternary strata generally 

 commences with a coarse gravel, a few feet thick, eating into the 

 chalk or London clay in which it lies, and mixing up the clay or 

 chalk with the gravel. 



The coarse gravel often passes into coarse sand, and is evidence 

 then that the movement of the water at that point was not suffi- 

 ciently rapid to transport gravel, or that there was no gravel present 

 to be deposited at that moment. 



I have not observed any finely bedded clay or loam in contact with 

 the surface of the chalk or London clay in the London basin ; and 

 this appears as if the movement of water over the surface of the clay 

 and chalk had been generally rapid at the commencement of the 

 Thames deposits. No doubt there are many cases where the first 

 deposit has been of fine materials, but I have not been able to meet 

 with them. 



"Where concavities in the chalk and London clay exist in the 

 neighbourhood of the Thames, and in its side- streams, and are so 

 situated as to be favourable for quiet deposition, the lowest bed of 

 gravel or coarse sand sometimes contains fossils derived from the 

 Eocene beds, and rolled Mammalian remains of a later period. It is 

 succeeded by a series of laminated clays and false-bedded sands, from 

 20 to 30 feet in thickness, with water-worn materials, and contain- 

 ing fluviatile and land shells, tranquilly deposited with Mammalian 

 remains (only occasionally rolled). Ko natural sections exist of this 

 class of deposits ; but excavations for brick-earth have during the 

 last 30 years opened out several of these fluviatile beds. 



Non-fossiliferous gravels are often clearly contemporaneous with 

 these fluviatile brick-earths. If on the same horizon, they are some- 

 times interstratified with the fossiliferous beds ; but the non-fos- 

 siliferous gravels reach to much higher levels, and are deposited 

 at much steeper angles, and contain materials not perfectly washed 

 or water-worn. They include masses of Thanet sands, of plastic 

 clay, of London clay, and of the fossiliferous bands of the Wool- 

 wich series, buried in great uni oiled masses in the gravel, as well 

 as Avater-worn masses of Druid Sandstone, derived from strata 

 very near the spot of deposition. These materials were evidently 

 washed in by heavy floods, not confined to the valleys, but passing 

 over the whole surface of the land, tearing up the ground and carrying 



