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 chalcedony. 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 beautifully the sponge markings,--a wide central canal with fine thread-like canals leading into it from all sides. The Chalk Sea evidently abounded in siliceous organisms, and it cannot be doubted that it is from such organisms that the silica was derived, which has formed the masses of flint. Silica occurs in two forms--in a crystalline form as quartz or rock crystal, and as amorphous, _i.e._, formless or uncrystalline (also called opaline) silica. The siliceous skeletons of marine organisms are formed of amorphous silica. Flint consists of innumerable fine crystalline grains, closely packed together. Amorphous silica is less stable than crystalline, and is capable of being dissolved in alkaline water, _i.e._, water containing carbonate of sodium or potassium in solution. If the silica so dissolved be deposited again, it is generally in the crystalline form. It seems probable, therefore, that the amorphous silica of the skeletal parts of marine organisms has been dissolved by alkaline water percolating through the strata, and re-deposited as flint.