WALTOX COMMON EXPOSIXG LONDON CLAY ETC. 153 



a certain amount of speculation. In the Survey map, which gives 

 the superficial geology of London and its environs, Lower Bagshot 

 Sand in situ is represented here ; and a fringe of Lower Eagshots is 

 shown all along the eastern margin of the great mass of Plateau- 

 gravel to which allusion has been previously made. When first I saw 

 this place it occurred to me for a moment that there might be a fault 

 here which had let down the Bagshots, so as to preserve them from 

 destruction, and also accounting for the singular position in which 

 the yellow sand is found in reference to the London Clay. Some- 

 thing of this kind there may be, yet it could hardly account for 

 everything on the supposition that the yellow sand is nothing more 

 than a mass of Lower Bagshot in situ. Besides, the lithology is 

 against this supposition. On the whole I. am rather inclined to 

 believe that the upper and more false-bedded portions of the yellow 

 sand have been reconstructed with a slight intermixture of foreign 

 material. The only portion, then, really in situ, on this supposition, 

 is the lowest part of the mass, beneath which no gravel can be 

 detected. It is perfectly obvious that the eastern end belongs to 

 the ''mixed series;" and perhaps the whole of the "yellow sand," 

 with the exception of the horizontally bedded sands at the base, 

 should be classed with the " mixed series" (cc'). 



This latter may be regarded as constituting a sort of passage 

 between the Plateau-gravel and the Thames-valley gravel — a view 

 which accords well with its position on the slope. If this be correcft, 

 it is just possible that the old margin of the Mole-valley inlet may 

 at one time have been at the spot where this reconstructed Bagshot 

 Sand now abuts against the steep surface of London Clay covered by 

 Plateau-gravel. The action of coast-ice, too, may have had some- 

 thing to do with the peculiarities to be noted hereabouts, and some 

 of the sand lumps in these gravels may have been frozen, and so 

 kept together ; but I am inclined to think that reconstruction by 

 water will account for most of what we see as regards the yellow sand. 



On reexamining this section in the early part of January 1886, 

 the improbability of this yellow false-bedded sand at the " corner," 

 repres'enting Bagshots in situ, as indicated in the Survey map, was 

 still more strongly impressed upon me, since the general structure 

 of the sand and the entire absence of the thin argillaceous layers is 

 so different from what obtains even in the basement-bed of the Lower 

 Bagshots. These remarks apply more especially to the upper part 

 of the deposit, where a singular appearance was noted towards the 

 surface, which I have endeavoured to represent in fig. 4. 



The following notes as to the character of the two varieties of 

 sand are appended : — 



A. The main mass or false-bedded series. The quartz grains are 

 irregular, and are often pebbles reaching a diameter of 3 millim. 

 Bleached flint chips 5 or 6 millim. in diameter are not uncommon. 

 It contains the elements of the basal sands of the Lower Bagshots, 

 together with rounded grains of dull quartz of the size before 

 mentioned. Resembles in many respects the sands of the Plateau- 

 gravel, but cleaner. 



