156 MR. W. H. HUDLESTON ON A SECTIOlSr THROUGH 



due to iron mineral. Glauconite granules were not observed ; but 

 there are a few angular fragments of a clear green mineral 

 substance. 



In the more sandy beds there is not quite such an immense pre- 

 ponderance of quartz, since we find more " glauconite," of a pale 

 greenish grey, and in some cases of a marked green colour. The 

 quartz is more frequently cubical in form, but rarely rounded ; 

 vitreous varieties predominate. The size of the granules is perhaps 

 from J^ to yL. millim. but with stray pellets up to -i- millim. 

 Within the above limit, there is considerable variety in the different 

 beds, more especially as regards the amount of " glauconite." In 

 the ribands of sand before mentioned the grains are quite clean, and 

 may be examined without washing. The sample is very similar to 

 the washings obtained from the sandy clays, but the general colour 

 of the mass is rather paler. 



The Plateau-gravel over tJie Lcnclon Clay. — At the "corner," 

 where everything is muddled up, pieces of discoloured sand may be 

 noted in the Plateau-gravel. As a rule, throughout the eastern 

 portion of the block, even including the " Top Sand," here very thin, 

 it does not exceed 6 feet in thickness, but further westwards is seen 

 to increase. The undulations in the London-Clay surface are com- 

 pensated by the irregular depth of the Plateau-gravel. Over the 

 clay area the proportion of sand is less. Pebbles and regular 

 bedding predominate in the lower portions ; the upper part of the 

 gravel is more messed, contains more angular flints, and is, on the 

 whole, more sandy. The downward action of solvents has caused 

 the lower pebbles to possess the strongest coating of iron oxide. 

 In some places there is a thin ferruginous pan at the junction with 

 the London Clay ; and in one place this has assumed rather consider- 

 able proportions. The higher pebbles and subangular flints are more 

 or less bleached. 



The preceding considerations lead us to conclude that the line of 

 London Clay on Walton Common, in maps showing the solid geology, 

 should be advanced westwards till within 30 yards east of the centre 

 of Sir Eichard's Bridge : whilst, if shown on the permanent way, 

 the boundary-line must be advanced considerably beyond that 

 bridge. 



Block C. (Fig. 5.) 



Though the shortest of the blocks, perhaps this is the most inter- 

 esting, since we here obtain evidence of the relations of the basal 

 beds of the Lower Bagshots to the London Clay. Fig. 5 is an 

 enlargement of that portion of block C which lies west of the centre 

 of the bridge. 



As the curve of the London-Clay surface sinks in a series of minor 

 undulations towards the west, traces of the lowest bed (No. 1) of the 

 Lower Bagshots may be noted about 30 yards east of the centre of 

 the bridge. It is a yellow sand, nearly 2 feet thick here, and ter- 

 minates with a very blunt point against the London Clay. At the 

 centre of the bridge the following section was observed : — 



