Ji2 FLINTS IN THE CHALK. 



pits of Charlton, Grays, Gravesend, and Caterham. Sometimes, as in 

 the neighbourhood of Leatherhead, the nodular flints have become sub- 

 sequently cemented together, and embraced in horizontal tabular masses 

 of flint. Another mode of occurrence of this substance is seen in the 

 chalk of Norfolk and of Antrim, where subcylindrical vertical masses, 

 like sacks of flour or sugar, are piled one above the other. These ver- 

 tical flints are commonly called " pot-stones," because as they stand up 

 on the beach near Cromer, they exhibit a tubular or pot-like appearance. 

 This form is nowhere better seen than at Horstead, near Norwich, 

 where the flints extend all round the pit in vertical columns, which 

 pass through the horizontal layers of nodules. Sometimes the external 

 surface is smooth, sometimes nodular and irregular. Each flint varies 

 in height from one to five feet, and is usually a foot or two in diameter. 



A third mode of occurrence of flint is well seen on the coast of 

 Sussex near Rottingdean, where fissures in the Chalk are filled with 

 tabulae of black flint, which are inclined to the stratification, and some- 

 times pass through layers of nodules. . 



The formation of flint has generally been attributed to the 

 growth, at the time the Chalk was forming, of organisms having 

 a silicious skeleton, such as Diatomaceae, Polycystinse, and sponges. 

 The flint in fissures, like a multitude of other phenomena, demon- 

 strates that its accumulation in the forms now seen is of later date 

 than the consolidation of the Chalk. In fact, flints in chalk have 

 grown like septaria in clays. The material was there from the 

 first, diffused in the rock, but it has taken all subsequent time 

 for it to assume its present conditions of aggregation. Dr. Car- 

 penter, impressed with the way in which sponges, like Holtenia, 

 radiate their silicious roots into the calcareous mud of the ocean in a 

 cylindrical figure, has suggested that the accumulation of silica from 

 the growth of successive generations of sponges, one above another, on 

 the same spot, might account for the nucleus around which the silica 

 of pot-stones became aggregated. At the present day the chalk mud 

 of the deep ocean includes in some localities from twenty to thirty or 

 forty per cent, of silica, chiefly due to the accumulation on the bottom 

 of silicious skeletons which fall from the surface ; and therefore, 

 unless they were especially abundant at particular periods of time, 

 the horizontal layers of nodules would have to be attributed mainly to 

 a similar extraordinary development of sponges on the ocean floor, for 

 unless the silicious matter had been deposited more freely in certain beds 

 of chalk than in others, it would have been impossible for the diffused 

 flint in the Chalk to have gathered about nuclei in the way observed 

 in the horizontal layers. Dr. Wallich believes that the flint was ac- 

 cumulated in the protoplasm of sponges, but there is no evidence 

 to confirm this view. It may, however, be remarked that the Upper 

 Chalk of Yorkshire which contains no flints has in it twice as much 

 silica as the middle or flint-bearing Chalk. Flint may occur sporadi- 

 cally, investing organisms of all kinds, or even in internal cavities of 

 teeth into which no organisms could have penetrated. 



