CAPPING CLAY. 239 



swells or "caps" of gravel, by Wimbledon, Clap- 

 ham, Brixton, and so onward till one comes to the 

 chalk formation near New Cross? So, also, the 

 heights of Baling, Kensington, Primrose Hill, 

 Hampstead, Highgate, Pentonville, and the other 

 swells, towards Finchley common and the flats on 

 the river Lea. It is true that where the estuary of a 

 river so meets the set of the tide as to form a con- 

 stant eddy in the waters, and a permanent whirlwind 

 in the air, hills of sand are in some instances col- 

 lected, as high as any that have been named, or 

 even higher. There are instances of them at the 

 mouth of the Tay, below Dundee ; and at that of the 

 Findhorn, below Forces ; and on some of the sandy 

 shores of the Continent. On that of Jutland, for in- 

 stance, they are very numerous, and formed without 

 any river, by the action of the sea-eddies alone. So 

 also in the sandy deserts, there are hills of sand 

 formed by whirlwinds or eddies of the atmosphere, 

 without any assistance from water, for there is no 

 water there. But these cases will not explain the 

 formation of the eminences in the valley of the 

 Thames. These contain flint pebbles, which are 

 rather too weighty for being built into hills by the 

 winds ; and they also contain beds of clay, a sub- 

 stance which imbibes too much water, and forms too 

 much in the state of a paste, for drifting much with 

 the winds. Besides, the "London clay" is obvi- 

 ously a gradual deposite from water which has stood 

 over the highest points where it is found ; and even 

 though we consider the flint gravel as the debris of 

 chalk rocks, out of which all the lime has been 

 washed except that which suffices to give a binding 

 quality to the gravel, we must allow it to have been 

 rolled about in the water till the flints abraded each 

 other into smoothness, and the dust thence produced 

 formed the connecting powder of the gravel. It is 

 impossible to say how long it may have taken to 

 round the nodules and produce the powder : but the 



