Chalk, an earthy mineral, consisting of carbonate of lime of friable texture, easily rubbed to a white powder. It constitutes rock formations of vast extent, being seen along the shores of the North sea and the English channel, in England and France, towering up in cliffs sometimes 1,000 ft. high, that dazzle the eye with their brilliant whiteness. The rock formation of which chalk is the principal member, and which is called the cretaceous or chalk formation, is the upper group of the secondary series. It is traced across the continent of Europe from the north of Ireland toward the southeast to the Crimea, and from the south of Sweden to beyond Bordeaux, occurring in patches over the greater part of the included area. It gives to the topography an interesting variety of abrupt cliffs upon the coasts and rivers, and of bold hills in the interior, intersected in every direction with valleys of smooth and flowing outline; but the soil it produces is in general too calcareous to he very productive. A remarkable feature in the chalk formation in some localities is the occurrence of layers of flint nodules in the rock, horizontally arranged, and not in contact with each other, and of all shapes and sizes, varying from an inch to a yard in circumference.

The flints i frequently appear to be concretions of silicious matter around organic substances, as parts of shells, sponges, etc, into the most minute pores of which the silica has penetrated, beautifully preserving their peculiar forms. The chalk itself is in great part composed of finely comminuted shells and corals, and it is now generally understood to have been derived from the same sources as the fine white calcareous mud which fills the bottoms of coral lagoons, and the interstices of its structures. This proves to be entirely of animal origin, in part finely ground shells and corals, and partly the excrement of shell fish, and of certain gregarious fishes, which in the coral regions of the Pacific were seen by Darwin through the clear waters, browsing quietly in great numbers upon living corals, like grazing herds of graminivorous quadrupeds. In the coral reefs of the south seas Prof. Dana found portions of these as compact and solid as any secondary limestone, and parts of the still growing structures not to be distinguished from portions of the chalk rocks of the cretaceous formation. The fossils of this geological group are all of oceanic families, but of extinct species.

Several species found in New Jersey are identical with those of the same formation in Europe; but the chalk is absent, though the other strata of limestone and greensand are recognized as those which elsewhere accompany it. - Chalk is employed for a variety of purposes. It is easily converted into lime, in which state it forms a valuable fertilizer as well as cement. It is used as a marking material, and also for polishing metals and glass. When finely ground, and purified by washing and separating its harder particles, it is sold by the name of whiting, or Spanish white. The flints found in the formation were once much used in England in the manufacture of glass. - In medicine, chalk, when thoroughly purified, is used, under the name of prepared chalk, as an absorbent in diarrhoea; it is also an antacid, and is used to furnish carbonic acid gas; it is also a dentifrice. Chalk mixture is the form in which prepared chalk is commonly administered medicinally. When diarrhoea depends upon irritation or inflammation of the mucous membrane of the bowels, it should not be employed, for it is then more likely to increase than allay the disturbance.

It may often be advantageously combined with opium or astringents, or both. - French chalk is a pure variety of steatite or talc, used by tailors for marking cloth, and also mixed with cosmetics to give them body. Black chalk is a variety of bituminous shale, made use of by artists for drawing. Red chalk, or reddle, is an argillaceous red oxide of iron.