Vast mountains of limestone occur in several countries of the globe; but no where is lime more abundant than in some parts of England and Wales. It forms, in particular, nearly the whole mountainous districts of Derbyshire and Shropshire, and encloses, in its substance, numerous veins of lead ore, calamine, and other important mineral productions. Its uses have been already described (139). 141. _CHALK is a white or yellowish kind of limestone, too well known to need any description._ It is found abundantly in many of the southern counties of England, and is usually procured from large open places, called chalk─pits, by digging. In some parts of Kent, however, the workmen save themselves, in this respect, much trouble. They undermine the sides of hills to a certain depth, then dig a trench at the top as far distant from the edge as the mining extends at the bottom. This trench they fill with water, which soaks through during the night, and the whole mass is thereby loosened, and falls down before morning. The harder and more compact kinds of chalk are cut into blocks, and used as building stones. When burned and formed into lime, chalk becomes an excellent mortar: nearly all the houses in London are cemented with chalk mortar. It is also used as lime in agriculture. As it readily imbibes water, it is used by starch─makers, chemists, and others, to dry precipitates upon. With isinglass or the white of eggs it forms a valuable lute or cement. By artists it is in request for the construction of moulds to cast metals in; and by carpenters and others, as a material to mark with. Chalk is one of the most useful absorbents that are employed in medicine: it likewise gives name to an officinal mixture, to a powder, and a potion. When pounded and cleared from gritty particles, it has the name of _whiting_. In this state it is used for the cleaning and polishing of metallic and glass utensils; for whitening the ceilings of rooms, and numerous other purposes. _Spanish white_ is the same substance cleansed with peculiar care; and the _Vienna white_, which is used by artists, is perfectly purified chalk. 142. _MARBLE is a compact and close─grained kind of limestone; so hard as to admit of being polished. It is this quality which principally distinguishes it from other calcareous substances._ Although nearly all the numerous kinds of marble may be burned, and thus converted into quick─lime, their use in ornamental architecture, &c. is so important as, in general, to prevent their application to the inferior purpose of mortar. Marble has been known from a very early period. The Book of Esther, in the Old Testament, describes the palace of Ahasuerus to have had “pillars of marble,” and the pavement of “red, and blue, and white, and black marble.”