WALTON COir.AIOX EXPOSING LONDON CLAY ETC. 155 



The following note is given as to the character of the dried claj'. 

 The London Clay of the Walton cutting breaks into greyish lumps 

 made up of finely granular matter with sparkling points. These are 

 mostly clean quartz, with perhaps a speck here and there of white 

 mica. These lumps crush into a fine grey and subangular powder. 

 There is considerable equality amongst the constituent granules, 

 which have an average diameter of -08 millim. The granules are 

 mainly quartz, but nearly always invested with a greyish white 

 kaolin-like substance, which breaks off in very thin flakes ; it is by 

 means of this substance that the aggregation of the quartz granules 

 is effected. As in the Lower Bagshots there is a moderate quantity 

 of green grains, both pale and dull green, always very small. Pieces 

 of an iridescent mineral, probably sulphide of iron undergoing 

 oxidation, may also be noted. 



One of the slides exhibited represents the very coarsest particles 

 after washing, and here a certain number of larger quartz grains 

 may be noted ; these are mostly rather rounded at the edges, and 

 present an exceptional feature as regards size. Amongst the thin 

 flakes of the kaolin-like substance are numerous specks of quartz 

 (much smaller than the grains), but notwithstanding their extreme 

 smallness in very good optical condition. This is shown in another 

 slide. 



When the section was very fresh the bedding of the London Clay 

 was almost invisible, and the clay looked like a mass entirely homo- 

 geneous. Weathering has subsequently developed the bedding, 

 especially of the upper layers, which, on the whole, are more sandy 

 than those of the base of the cutting. It is only in a few places 

 that the bedding is visible. For instance, wherever the sides of 

 the cutting have been sloped, the sliding forward of the superin- 

 cumbent gravel has effaced every feature ; but there are some por- 

 tions towards the centre of block B where the bedding is very well 

 seen. The occurrence of ribands of light-grey sand, near 2 inches 

 in thickness, helps to bring it out more distinctly. Where visible 

 there is a dip of about 1°, or rather less, down the line, i. e. towards 

 the west. If this is continued the beds at the junction with the 

 Bagshots must be higher in the series than those in the middle of 

 block B. Besides the ribands of sand already mentioned, lenticular 

 patches of small extent may be noted. The deficiency of calcareous 

 matter is probably the cause of the absence of Septaria throughout 

 the London Clay of this cutting. 



A further microscopic examination of the sand-grains in the 

 London Clay shows that angular quartz, mostly of a highly vitreous 

 kind, immensely preponderates over all other constituents, which 

 latter may be regarded more or less as mere accessories. In the 

 clayey or unctuous varieties these grains are often more like quartz 

 chips or splinters, and range from -^^-^^ to J^ millim. in diameter, but 

 with an admixture of a larger size, from i to y^^ millim., which 

 were originally more cubical in shape, and are sometimes a little 

 rounded. This seems to point to two independent sources for the 

 material of the deposit. In this variety the black specks are mostly 



