1862.] WHTTAKEE— LONDON BASIN. 265 



of the Bracklesham (or Middle Bagshot) Beds ? — if so, the Lower 

 Bagshot Sand must be thin here. 



A little to the north-west of the northern end of the above outlier 

 there is a patch of sand, thickly overgrown with wood. 



At Leigh Hill is another sand-outlier, the boundary of which is 

 not quite clear. There are some very large and fine Scotch-firs on 

 and near this the most western mass of Lower Tertiary beds in the 

 London Basin. 



Surface-deposits on the Ohalk of this District. — These are of two 

 sorts — the more widely spread being a stiff clay of a brown or red 

 colour with angular flints, which I term " Clay-with-flints * ; " the 

 other and more valuable one being a loam or sandy clay of various 

 colours, mostly fit for making bricks of, and known therefore as 

 " Brick-earth." 



The Clay-with-flints lies very irregularly on the Chalk, for the 

 most part filling pipes in that rock. The Brick-earth is generally 

 underlain by the clay. 



As there is no Survey-memoir illustrating Sheet 14, it wiU be well 

 to note here the range of these surface-beds, which were at first 

 mapped and published as Eocene in that sheet. The Clay-with- 

 flints rarely occurs on the top of the great Chalk-escarpment over- 

 looking the Yale of Pewsey ; but it covers the Chalk over nearly the 

 whole of the higher grounds from the eastern part of the district 

 westward to near East Kennet — ^not, however, in one continuous 

 sheet, but forming many separate patches. The Tertiary beds are free 

 fi'om it ; indeed the Clay-with-flints does not seem to occur else- 

 where than on the Upper Chalk, as I have before noticed (in the Geo- 

 logical Survey Memoir on Sheet 13, p. 55). 



Over this widespread bed of clay there is here and there a mass 

 of the more sandy Brick-earth. Near Tevals Farm, about two miles 

 S.S.W. of Marlborough, there is a brickyard ; there is another by the 

 turnpike-road about a mile S.E. of the same town; and a third on 

 the west of Hens Wood, some three nules to the E.N.E. Without 

 doubt there are many other masses of Brick-earth, which perhaps 

 may be too thin or too full of pebbles to be worked. The bricks made 

 from this bed in the neighbourhood of Marlborough are remarkable 

 for their beautiful rich crimson colour, as may be seen in many of 

 the buildings in that town. These surface-beds are not marked by 

 features as the outliers of true Tertiary beds are for the most part. 

 Thus whilst the latter rise from above the surrounding Chalk, the 

 former merely fill hollows in that rock, and have only been saved 

 from denudation by their sheltered position. As to their age and 

 origin I do not feel able to give an opinion with any certainty. 



Part II. — This part does not refer to structure, but simply to 

 thickness ; in it I shall make use largely of the sections given by 

 Mr. Prestwich in his papers '^ On the Thanet Sands f," " On the 



* See Memoir illustrating Sheet 13 of the Map of the Q-eologioal Survey, 

 p. 64. 

 t Quart. Joum. Geol. Soc. vol. viii. p. 235. 



