The flooding of streams that flow through chalky ground is almost unknown, since chalk is very porous, and will when dry take up nearly one-third of its bulk of water (Ansted and Miller); and in some districts where chalk is the surface-rock, the heaviest showers are absorbed as soon as they fall. During the summer months, however, most of the rainfall penetrates only a few feet downwards, and is consequently imbibed by vegetation or evaporated; for though chalk is highly absorbent, its fine and close texture does not allow of the free permeation of water. Occasionally, lower-lying beds of chalk have been drained, whilst higher levels have remained charged with moisture. The circulation of water in chalk strata is due, in fact, not so much to general percolation in the mass, as to its flow either through fissures or, as in the case of deep-seated and but little disturbed and creviced beds, along planes of stratification. As a storehouse of water the chalk formation is of great importance, the artesian wells of London and many other places being supplied directly or indirectly from it. Where chalk forms the surface of the land, the yield of water may be very large, as at the Tring cutting of the North-Western Railway, where it amounted to upwards of a million gallons a day. Chalk is much used as a dressing for land deficient in lime, and is of special value where the soil consists of a stiff wet clay. The effect of the addition of chalk is to render the soil better able to retain moisture, and to improve its texture. It is generally most efficacious when used in moderate quantity every six or seven years. The renewal of the supply of chalk is necessary owing to its removal from the surface by solution and by the undermining action of earthworms. The chalk is not ploughed into the land till it has been exposed to frost, which breaks it up into small squarish fragments. In the Lincolnshire wolds, the process of chalking, in conjunction with boning and manuring, has brought about a great improvement in the soil. In Dorsetshire the land is usually chalked once in twenty years. The chalk is spread on the ground in large lumps, which crumble down under the influence of the weather; or it may be burnt and applied as lime. For sandy soils, oh'which it acts as a cement, chalk is best adapted when marly in character. On poor chalky ground, the sainfoin (Onobrychis sativa), a plant which flourishes in calcareous soils and is an excellent fodder for cattle, has been cultivated with great success; and the herbage of the chalk downs of Sussex, Wilts, and other counties of England, affords good pasture for sheep.

Chalk is employed medicinally as an antacid and mild astringent, either alone, or together with other astringents, or combined with mercury as hydrargyrum cum creta, but never in the crude state. As none of the salts it forms is purgative, it is a valuable remedy in diarrhoea. Externally, prepared chalk is used as a dessicant, and as a protective application for ulcers and burns, and in intertrigo and other affections of the skin. When taken for a prolonged period, chalk is apt to form concretions in the stomach, for want of acid sufficient to dissolve it.

Chalk is rarely hard enough to be used in any kind of building. When burnt to lime it serves the same purposes as stone lime, but on account of its more rapid absorption of carbonic acid gas, it is not so fit for general use as the latter, except when fresh from the kiln. Chalk is employed in the manufacture of cements and of carbonate of soda, in the preparation of carbonic acid gas, and in many other chemical processes; also for making paints, crayons, and tooth-powder. Whiting or Spanish white, used to polish glass and metal, is purified chalk prepared by triturating common chalk with a large quantity of water, which is then decanted and allowed to deposit the finely-divided particles it holds in suspension.

Black chalk or drawing slate is a soft carbonaceous schist, which gives a black streak, so that it can be used for drawing or writing. Brown chalk is a kind of umber. Red chalk or reddle is an impure earthy variety of haematite. French chalk is a soft variety of steatite, a hydrated silicate of magnesium.

See J. Prestwich, The Water-bearing Strata of London, p. 57, sqg. (London, 1851); J. A. Clarke in Journ. Roy. Agrw. Soc., vol. xii. p. 331 (1851); Caird, English Agriculture in 1850 and 1851, p. 61 (London, 1852).