Chalk

Chalk, an earthy form of limestone, belonging mostly to the upper part of the Cretaceous system (q.v.). It is usually white, but sometimes grey, red, or greenish; is very friable, breaking with an uneven fracture, and has an insipid taste. In addition to carbonate of lime (CaCO₃), chalk usually coutains some silica, iron-oxide, and alumina, the Red Chalk of Hunstanton, in Norfolk, containing more than nine per cent. of these impurities. On microscopic examination chalk is found to consist mainly of the shells of Globigerina and other Foraminifera (q.v.), thus presenting some analogy to the white ooze now forming in the deep Atlantic. Chalk occurs in thin beds under the Tertiary volcanic rocks of Mull, Morven, and Antrim, where it is baked so as sometimes to have become marble (q.v.). From the western Midlands it has been removed, leaving behind its less destructible flints, and now extends as an escarpment from the borders of Devon and Dorset, through Salisbury Plain, the Hampshire and Berks downs, the Chiltern and Gogmagog hills, and the wolds of Norfolk and Lincoln, to Flamborough Head. A downward fold carries it under the London basin, and parallel upward curves form the North and South Downs, ending in the Forelands and Beachy Head, and the downs of the Isle of Wight. On the Continent it extends to the Crimea and from the south of Sweden to Bordeaux, over about 1,100 miles in one direction, and nearly 900 in the other. It varies in thickness from 600 to 1,000 feet, and its general freedom from sand and pebbles points to its formation in open sea, though masses of granitic rocks, believed to have been carried by ice, have been found in it. Besides the minute foraminifers, the most characteristic fossils of the Chalk are sponges, often in the flint-nodules that occur in fines in the Upper Chalk, sea-urchins in great numbers, brachiopods, such as Terebratula (q.v.), scallops, oysters, the bivalve genus Inoceramus, ammonites, the teeth of sharks and rays, and the bones of turtles and saurian reptiles. Some of these being confined to certain parts of the formation, it has been divided into zones which are grouped in three main divisions, bearing names of French origin - the Cenomanian, impure and marly, including the Grey Chalk of Folkestone and Totternhoe stone; the Turonian, or Lower Chalk without flints, surmounted by the Chalk Rock of Dover; and the Senonian, or Upper Chalk with flints. Chalk, being very porous, is an important water-bearing formation, all the deep wells of London being sunk into it. The water is hard, but softens on boiling. Chalk forms a poor soil, but is generally covered with "clay with flints," a ferruginous clay residue from dissolved upper layers, which bears a close turf suitable for sheep-grazing, or is fitted for root-crops. Chalk is largely used for dressing clay lands, for lime-burning or cement-making, and in making carbonate of soda, carbonic, acid, paints, and tooth-powder. Whiting is simply fine pure chalk triturated with water and re-precipitated. Chalk is used in medicine as a desiccant and as an antacid. French chalk is an entirely different substance, an earthy form of talc (q.v.), and forms the basis of rouge and of some coloured crayons.