Chalk
a well-known earthy limestone, of an opaque white color, soft, and admitting no polish. It is an impure carbonate of lime, and is used as an absorbent and anti-acid, and for making marks for various purposes, as on the blackboard in schools, and by artisans and others. Black chalk is a soft variety of argillaceous slate. Brown chalk, a familiar name for umber. Red chalk, another name for ruddle. French chalk, steatite or soap-stone, a soft magnesian mineral. Drawing Drawing chalks were origioriginally restricted in colors to white, black, and red, but now chalks of every color are used, and are known by the name of crayons, commonly made of calcium sulphate and not the carbonate, as supposed. In geology chalk is the rock which forms the higher part of a series or group of strata, comprising rocks of different kinds, termed the cretaceous system. a