",,. CAPPING CLAY. 251 



of gravel, by Wimbledon, Clapham, Brixton, and 

 so onward till one comes to the chalk formation 

 near New Cross ? So, also, the heights of Ealing, 

 Kensington, Primrose Hill, Hampstead, Highgate, 

 Pentonville, and the other swells, toward 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 constant eddy in the 

 waters, and a permanent whirlwind in the air, hills 

 of sand are in some instances collected, 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 Torres ; and on some of the sandy shores 

 of the Continent. On that of Jutland, for instance, 

 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 wa- 

 ter, for there is no water there. But these 

 cases will not explain the formation of the emi- 

 nences in the valley of the Thames. These con- 

 tain flint pebbles, which are rather too weighty 

 for being built into hills by the winds ; and they 

 also contain beds of clay, a substance which im- 

 bibes too much water, and forms too much in the 

 state of a paste, for drifting much with the winds. 

 Besides, the " London clay" is obviously a gradual 

 deposit 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 



