CHALK, in natural history, a species of CALK, which see.
Chalk, where it is found at all, is the preponderating substance, and may there fore be considered as characterizing a peculiar species of mineral formation. It is perhaps the most recent of all the va rieties of calcareous carbonates ; it occurs in strata for the most part nearly horizon tal, alternating with thin layers of flint nodules, and with the same irregularity dispersed through its substance ; it con tains in abundance the relics of marine organized bodies, such as echinites, glossopetrx, pectinites, &c. and also not unfrequently the hard parts of amphibi ous and land animals, as the heads and vertebra of crocodiles, and teeth of ele phants. Chalk hills never rise to a high er elevation than three or four hundred feet, and are at once distinguishable by the smooth of their outline, and their remarkable tendency to form cup-shaped concavities. Ridges of chalk, in England at least, are always bordered by parallel ranges of sand or sand stone, beneath, and alternating with which are situated the beds of fullers-earth. Chalk hills are also singularly characterized by their dryness and their verdure :the most porous sand-stone is scarcely so deficient in springs of water, and yet, except upon almost perpendicular descents, the white surface of the chalk is uniformly covered with fine turf or wood.
The chalk hills in England occupy a greater extent than in any other country ; they run in a direction nearly from east to west, parallel to each other, and sepa rated by ranges of sand-stone, and low tracts of gravel and clay. The most north ern and loftiest range of chalk com mences at the promontory of Flambo rough-head, in Yorkshire, and proceeds westward for nearly twenty miles. In the
county of Lincoln are some fragments of a ridge near Grantham. Two ridges tra verse the midland countries, and reach as far west as the borders of Oxfordshire : these ridges are no where so conspicuous as in the county of Bedford, where they approach near to each other, being only separated by the Woburn and Ampthill range of sand-stone. The country south of the Thames also contains two ridges, the one commencing at the North and South Foreland, passing through the north of Kent, the middle of Surry, and the north of Hampshire, and including the North Downs of Banstead, Epsom, &c.: the other, commencing near Hast ings and at the lofty promontory of Beachy.head, passes through Sussex and the south of Hampshire, into Dorsetshire, including the South Downs. The north part of France also abounds in chalk : it is besides met with in some of the Danish islands in the Baltic, and in Po land.
The uses of chalk are very extensive : the more compact kinds are used as building stone, and are burnt to lime (nearly all the buildings in London being cemented with chalk-mortar :) it is also largely employed in the polishing of me tals and glass, in constructing moulds to cast metal in, by carpenters and others as a material to mark with, and by starch Makers and chemists to dry precipitates on, for which it is peculiarly qualified, on account of the remarkable facility with which it absorbs water.