THE CHALK 47 



also shattered the flints in the chalk into fragments. The 

 rounded masses retain their form, but when pulled out of 

 the chalk fall into sharp angular fragments, and we find 

 they are shattered through and through. 



Now, what are flints, and how were they formed? 

 Flints are a form of silica, a purer form than chert, as the 

 chalk in which they are embedded was formed in the deep 

 sea, and so we have no admixture of sand. Flints, as we 

 find them in the chalk, are generally black translucent 

 nodules, with a white coating, the result of a chemical 

 action which has affected the outside after they were 

 formed. Flint is very hard, harder than steel. You 

 cannot scratch it with a knife, though you may leave a 

 streak of steel on the surface of the flint. This hardness 

 is a property of other forms of silica, as quartz and chalce- 

 dony. The question how the flints were formed is a 

 difficult one. As to this much still remains obscure. 

 The sea contains mineral substances in solution. Calcium 

 sulphate and chloride, and a small amount of calcium 

 carbonate (carbonate of lime) are in solution in the sea. 

 From these salts is derived the calcium deposited as 

 calcium carbonate to form the shells of the Foraminifera 

 and the larger shells in the Chalk. There is also silica in 

 small quantity in sea water. From this the skeletons of 

 radiolaria and diatoms and the spicules of sponges are 

 formed. Now, many flints contain fossil sponges, and 

 when broken show a section of the sponge clearly marked. 

 Especially well can this be seen in flints which have lain 

 some time in a gravel bed formed of flints worn out of the 

 chalk by denudation. Hard as a flint seems, it is penetrated 

 by numerous fine pores. The gravel beds are usually stained 

 yellow by water containing iron, and this has penetrated by 

 the pores through the substance of the flints, staining them 

 brown and orange. Many of the stained flints show beauti- 

 fully the sponge markings, a wide central canal with 

 fine thread-like canals leading into it from all sides. 



