Yol. 62.] THE CLAY-WITH-FLINTS. 13? 



and a portion of this I have washed and examined under the micro- 

 scope, with the following result : — 



The greater part of the residue consists of rounded grains of clear white 

 quartz, some of them stained yellowish by oxide of iron, the grains varying in 

 size from very small to rather large. There are also many black and dark- 

 brown grains, which seem to be rolled particles of ironstone or small concretions 

 of oxide of iron. Both these ingredients have evidently been derived from 

 Reading Beds. A few fragments of Inoceramus-shell, with a few broken 

 sponge-spicules and arenaceous foraminifera, seem to be contributions from the 

 Chalk. Small angular chips of flint are common in the residue, but not in 

 the finer portion mounted for microscopic examination. 



A sample from near Eisborough, sent by the Rev. E. C. Spicer, 

 F.G.S., yielded a very similar residue, the only noticeable difference 

 being that there was a larger quantity of very fine quartz-sand, 

 large grains being rare. 



The residue of a sample from Chaul End, near Luton, sent to me 

 by Mr. J. Saunders, also consisted mainly of very fine red sand, 

 which the microscope showed to be grains, presumably of quartz, 

 coated with red clay. In all cases it was difficult to free the 

 mineral grains from the fine sticky clay which adhered to them. 



III. Thickness of the Clay- with- Flints. 



For the purpose of this enquiry, it is also necessary to form some 

 idea of the average thickness or mass of the Clay -with -Flints ; but 

 this is not easy, because the clay seldom occurs as an evenly-spread 

 deposit. Qn the contrary, it usually rests on an extremely-irregular 

 surface of chalk, having sunk into hollows, funnels, and pipes which 

 have clearly been formed by the unequal solution of the chalk. The 

 bottoms of the hollows and depressions are often 6 or 8 feet below 

 the summits of the intervening pinnacles of chalk, while in some 

 places large basins occur, lined by Clay-with-Flints and filled with 

 masses of brickearth and gravel, from 40 to 50 feet in depth ; else- 

 where, narrower funnels and pipes filled with the same materials 

 penetrate the Chalk for 30 or 40 feet. 



But although, in many places, it cannot be said that the Clay- 

 with-Flints has any average thickness, there are limits within which 

 it varies ; and there are some localities where its base is not too un- 

 even to prevent a rough estimate of its average thickness from being 

 made by eye. In other words, one can form some idea of what its 

 average thickness would be if it were spread out on an even base. 



In the first place, I will quote some recorded observations and 

 estimates of thickness : — 



' At Oakridge, west of Wycombe Abbey [Bucks], there is a thickness of 

 between 4 and 5 feet of stiff reddish Clay-with-Flints, in places sandy and 

 pebbly.' T (H. B. W.) 



' In a new road-cutting west of Hemel Hempstead Clay-with-Flints was seen, 

 with a depth of up to 8 feet, resting irregularly on soft chalk with many flints.' 2 



' Round St. Albans this irregular deposit varies from a few inches to 4 or 

 5 feet in thickness.' 3 



1 W. Whitaker, ' Geology of London ' Mem. Geol. Surv. vol. i (1889) p. 286. 



2 A. G-. Cameron, in op. cit. p. 287. 



3 H. B. Woodward, ibid. loc. cit. 



