CHALK is a white or greyish, loosely coherent kind of lime stone rock, composed almost entirely of the calcareous remains of minute marine organisms (foraminifera, cocoliths, etc.) and frag ments of shells. The purest kinds contain up to 99% of calcium carbonate in the form of the mineral calcite. Silica is always pres ent in small amounts as the mineral opal, representing the remains of other minute marine organisms and sponge spicules. Minute grains of quartz, felspar, zircon, rutile and other minerals are also present. With the admixture of clayey material there may be an insensible gradation from pure chalk to chalk-marl. Not only may there be considerable variation in the composition of chalk (phosphatic, glauconitic, etc.) but there may also be wide vari ations in the colour (snow-white, grey, etc.) and the texture of the material. It may be soft, incoherent and porous or quite hard and crystalline.
The uses of chalk are numerous ; for example, as a writing material in the form of white and coloured crayons, for the manufacture of quicklime, mortar, Portland cement (see CE MENT), plaster and as a fertilizer. Whiting is prepared by grind ing chalk and collecting the finer sediments from water; this is used for polishing, making putty and many other purposes. Under the name of "Paris White," chalk is used in the manufacture of india-rubber goods, oil-cloth, wallpaper, etc. The harder kinds are extensively used as a building stone.
Large quantities of chalk are quarried in England in Kent, Sur rey, Sussex, Cambridgeshire, Lincolnshire and Hampshire, Kent being by far the most important. The production of English chalk is not far short of five million tons annually. A certain amount of this is exported to the United States, though there are extensive beds of chalk in Kansas, Arkansas and Texas.