Flint, a form of silica (Si02), almost pure, occurring in thin beds (" tabular flint"), or more commonly in layers of irregularly-shaped nodules or "flints," in the Upper and Middle Chalk. The phthanites of the Carboniferous limestones, and the cherts of the Purbeck and other beds, closely resemble flint; but are seldom as pure and have generally a splintery fracttere instead of the shell-like or "conchoidal" mode of breaking characteristic of true flint. Flint, consists of a mixture of crystalline and insoluble with non-crystalline and soluble silica: the exterior of the nodules is generally less dense and white; the interior horn-like, grey or black and homogeneous, or sometimes banded like an agate; and the centre often hollow and lined with minute crystals of quartz. They frequently have fossils of various kinds imbedded in them, sea-urchins, brachiopods, bivalves, etc., and may have their form entirely determined by branching sponges now represented by a mass of chalk in their central cavity. The microscope also reveals numerous spicules of sponges throughout the flint. No theory of flint formation is altogether satisfactory; but apparently the silica was removed in the first instance from sea-water by the action of living sponges and other organisms, and has subsequently been to some extent aggregated by a purely chemical process of concretion and replacement. It is noticeable that there is less diffused silica in the chalk nearest to the nodules, and that tabular flint occurs in vertical joints (q.v.) as well as along the horizontal beds of the chalk. These facts show aggregation subsequent to the deposition of the chalk. Flints are used for building, are broken up for road-metal, and are powdered and calcined for the manufacture of glass and pottery. In prehistoric times they formed the main material of all weapons and implements over most of Eeerope, knives, axes, hammers, hide-scrapers, and light-strikers. The earliest human period or Stone Age is subdivided into two, the Paleeolithic and Neolithic, distinguished by the" flints being merely chipped or being also polished. Though no longer used with iron pyrites (q.v.), and but seldom with steel, for striking a light, large numbers of flints are still chipped or "knapped" into gun-flints in Suffolk for use by African tribes, who prefer them to percussion caps.