FLINT, a mineral which may be regarded as a variety of quartz, or as intermediate between quartz and opal, consisting almost entirely of silica, with a very little lime, oxide of iron, water, carbon, and sometimes even traces of organic matter. It has a flat shell-like fracture, is translucent or semi-transparent, and varies in color from a very dark broWn, or almost' black, to light_hrown, red, yellow, and grayish white, and is sometimes veined, clouded, marbled, or spotted. Dark-colored Flints are most common in the chalk, in which principally F. occurs imbedded, forming nodules of various sizes, sometimes large nodular masses, of irregular and often grotesque shape; but ,gravel formed of ligh•colored flints is very common, and it is disputed whether or not a change of color has taken place by exposure to atmospheric and other chemical agencies. F. is sometimes found in beds, or veins. It is abundant wherever chalk formation extends, in England and other countries; rolled F. nodules are also often, found in com pound rocks, and in alluvial sells; vast alluVia tracts being sonielliues full of them. F.
geodes often contain crystals of quartz. F. nodules are usually moist in the interior if broken when newly taken from their beds.
F. is sometimes harder than quartz, sufficiently so to scratch it. The readiness with which is strikes fire with steel is well known, and it would seem that the sparks are not all merely incandescent particles, heated by the friction, but that in some of them a chemical combination of silica and iron takes place, causing great increase of heat. The use of the F. and steel for igniting kinder, once so common, has been almost super seded by that of lucifer matches, and gun flints have given place to percussion caps. According to Pliny, Clias was the first who struck fire with F. ; or more probably, he was the first to show its application to useful purposes; and he therefore received the name Pyrodeg. The most ancient use of F. was probably for sharp weapons and cutting
instruments; and F. knives, axes, arrow-beads, etc., are among the most interesting relics of rude antiquity.
At present, a principal use of F. is in the manufacture of fine earthenware, into the composition of which it enters, being for this purpose first calcined, then thrown into cold water, and afterwards powdered: The origin of F. is a subject of considerable difficulty. Siliceous deposits are some times a purely chemical operation, as in the case of the siliceous sinter formed round the geysers of Iceland, from the evaporation of water largely charged with sile±. But at the bottom of the sea, as no evaporation could take plate, some other agent than springs of water saturated with silex must have supplied the materials. It is a fact of considerable importance in this inquiry, that almost all large masses Of limestone have thin siliceous concretions, or flints. Thus. chert is found in carboniferous and other' limestones, and menilite in the tertiary -limestones of the Paris basin, The conditions necessary for the deposition of calcareous strata seem td be those required for the for mation of siliceous concretions. The materials of both exist in solution in sea-water, and as it needed the foraminifer, the coral, and the mollusk to fix the carbonate of lime which formed the chalk deposits, so the silex was secreted by innumerable diatoms and sponges, and their remains most probably supplied the material of the flint. The dis covery by Dr. Bowerbauk and other microscopists of the spicules of sponges and the frustules .of diatoms in almost- every specimen of F., has clearly shown that F. to a large extent, if not entirely, owes its origin to these minute organisms. It is, however, difficult to account for the changes that have taken place in these materials subsequent to their deposition.