CHALK (AS. cede, Ger. Ealk, Fr. chaux, Ir. mile. from Lat. cal.r, lime). A soft earthy vari ety of limestone or carbonate of lime, which often forms beds of great thickness and extent. lt is usually yellowish or white in color, has an earthy texture, is rough to the touch, and ad heres slightly to the tongue. The average chemi cal composition is as follows: (1) English chalk —calcium carbonate, 97.90 per cent.: silica, .66 per cent.; magnesium carbonate, 0.10 per cent.; ferric oxide. 0.35 per cent.; (2) Arkansas chalk— calcium carbonate, 95.29 per cent.; silica, 4,12 per cent.; alumina, 2.21 per cent.; ferric oxide, 2.21 per cent.
Chalk is formed by the acenmulation on the ocean bottom of the tiny shells of Foraminifera, fragments of molluscan 5: hells, sea-urchins' spines, and sponge-spicules. and when a thin sample of it is examined under the miscroscope it is an object of great beauty. lt is very abundant in certain areas and also in certain geological formations, the Cretaceous system being so named on account of the thick and extensive deposits of chalk which it contains. These de posits are especially abundant in England and France, where the great white cliffs of chalk on both sides of the English Channel. notably in the vicinity of Dover, England, and Dieppe, France, are conspicuous objects. Under London
the chalk formation has a thickness of from 600 to SOO feet. In the States chalk is known to occur in Iowa, Texas, and Arkansas, and other States.
Chalk is sometimes used as a building-stone, for. while it is sufficiently soft when fresh to be sawn, it often shows a tendency to harden on exposure to the air. It is burned to lime and the mortar is used generally in the construction of buildings in London. In England chalk has been used for many years as an ingredient of Portland cement, and more recently the Arkansas chalk has been employed for the same purpose. When ground and mixed with water it forms 'whitewash.' If the ground material is freed from grit by washing. it is known as 'whit hr.,' which is used for cleaning silver and making putty. 'Cilder's white' and 'Paris white' are forms of the same material which have been more carefully washed. On account of its color and softness, it finds general application as a marking material, but when so used has to be mixed with some bonding substance. (See CitAvoN.) Like lime and gypsum, chalk is used in fanning to ameliorate the texture of the soil. In a purified condition it is administered as medi cine to correct :oddity of the stomach. See CR•