AT THE BASE OF THE THANET SAND. 759 



than those of the southern bed. Under these circumstances it is 

 difficult to compare the flint-percentage. The flint forms about 10 

 per cent, of the quartz and flint grains, leaving the glauconite out of 

 consideration. This, though much lower than in the soutli, is still high. 

 The quantity of sand other than glauconite in the specimen brought 

 away was so small that it did not seem worth while to try to make 

 a separation ; but in slides of the sand left when the glauconite was 

 washed off zircon, rutile, tourmaline, black mica, and fragments 

 of the isotropic mineral described as garnet have been found. 

 The points of resemblance to the southern basement-bed of the 

 Thanet Sand are the facts that both are glauconitic, and contain 

 a larger proportion of flint than is common, as well as frag- 

 ments of a colourless garnet, which do not seem to be of such 

 universal distribution as the other heavy minerals common to both. 

 The basement Woolwich bed, though almost as largely glauconitic 

 where it rests on the chalk in Hampshire, difl'ers in several respects. 

 Its glauconite is of a blue and not a yellow green ; and though 

 search has been made, no flint grains have been found. 



The statement has been made in the Survey Memoirs that there 

 seems to be no proof of unconformity between the Chalk and the 

 Tertiaries. Prof. Prestwich, in his new volume, assumes such an 

 unconformity, since he says, " as the area of the Chalk-sea at the 

 close of the Cretaceous period gradually became more and more 

 restricted during emergence, so the early Eocene strata during the 

 first period of the following submergence were of very limited 

 extent " *. Although a small flint-percentage might be due to an 

 unconformity at a distance, so large a percentage could hardly have 

 occurred in a sand formed far from the source of the flint ; because 

 the further the flint was carried, the greater would be the chance 

 that, when deposited, it would be mixed with sand from other 

 coasts. If such a sand could only be formed close to a chalk-shore, 

 its existence at the base of the Tertiaries forms an additional piece of 

 evidence in favour of the gradual extension of the early Tertiary sea 

 described by Prof. Prestwich. 



One at once wonders how so large a flint-percentage could have 

 been formed in early Tertiary times, whilst the sand now being 

 formed along a very similarly situated shore contains little or no 

 flint. The diff'erence may, perhaps, be due to a difference in the 

 nature of the coast. Our coast consists of chalk-cliffs with the two 

 long breaks of the Tertiary and the Wealden sands and clays. Is it 

 not probable that currents drift the debris of these coasts as well as 

 the material brought down by the Thames to mix with the debris of 

 the chalk, and so hring down the flint percentage? If the early 

 Thanet- sea stretched from the borders of Belgium as far north as 

 Sudbury it would almost certainly have had something like 200 

 miles of unbroken chalk-cliff along its western shore, for the 

 Tertiaries were not there, and even Prof. Prestwich, who seems to 

 date the Wealden and Boulonnais anticlinal earlier than any one 



* ' Geology : Chemical, Physical, and Stratigraphical,' vol. ii. p. 337. 



