LOWER 7. Lower Grunsand 850 8. Wealden beds 1300 1. The maestricbt beds (q.v.) consist of pistolitic limestones in the n. of France, and of loose yellowish sandstones in Holland. 2. The chalk with flints is a great mass of pure white pulverulcnt limestone, usually too soft for a building-stone, but sometimes passing into a more solid state. It occurs in beds of great thickness, with the stratifica• . Lion often obscure, except when rendered distinct by mterstratified layers of flint a few inches in thickness, occasionally in continuous beds, but oftener in nodules, and recur ring at intervals from 2 to 4 ft. distant from each other. Iron pyrites is found frequently in these beds in radiated nodules; it readily decomposes, and produces rusty stains on the rock. 3. Chalk without flints; this differs from the upper chalk only in the want of flints. 4. Chalk marl; the white chalk, by the gradual admixture of argillaceous matter, becomes hardened, until it passes into a pale buff-colored marl or argillaceous limestone, sometimes of a sufficient compactness to be used as a building-stone. 5. Upper greensand (see GREENSAND), composed of alternating layers of sands, clays, and limestones, occasionally, but not always, colored with green particles of a chloride mineral. 6. Gault (q.v.), a stiff dark clay, used for brickmaking, with many beautifully preserved shells. 7. Lower greensand (q.v,), so like petralo,,oieally to the upper green sand, that when the intervening Gault is absent, it is impossible to separate them, except by their organic contents. The Specton clay, a local Yorkshire bed of dark clay, is of the same age. 8. Wealden (q.v.), divided into the two groups, the Wealden clay and Hastings sand, consists of a great series of shales and sandstones, with scattered beds of lime and ironstone.
The most remarkable petralogical characteristic of the group is the chalk, which exists in such abundance as to have given its name to the formation (Lat. ereta, chalk). It is a white, soft, and pulverulent limestone, consisting almost entirely of carbonate of Pine; the only foreign matter in any quantity being silex, which is aggregated together in an amorphous condition, in nodules or layers of flint. Occasional pebbles are also found, but they are extremely rare. Chalk was formerly supposed to be a chemical precipitate: the microscope has, however, shown it to be composed of minute shells mixed with the broken fragments of larger ones; and, very recently, the use of an im proved deep-sea sounding apparatus has revealed a sediment now accumulating in many places, which agrees in every point, save solidity, with the chalk. When a piece of white chalk is rubbed down to powder with water, by means of a soft brush, and the powder examined by the microscope, it will be found that the greater portion consists of shells of the minuter kinds of foraminifera, mixed with the disintegrated prisms of pima or other large shells of like structure, the shells of cytherina, a marine entomos traean, and probably a few diatoms. Deep-sea soundings have disclosed a formation
precisely similar, as taking place at the present time. Of some gatherings obtained at a depth of 2 m. from the great Atlantic plateau, prof. Bailey says: " I was greatly delighted to find that tel these deep soundings are filled with microscopic shells; not a particle of sand or gravel exists in them. They are chiefly made up of perfect little calcareous shells (foraminifera), and contain also a small number of siliceous shells (diatomacex)." The occurrence of pebbles in the chalk can easily be accounted for, if we suppose them to have been floated in, attached to the roots of trees, or more prob ably to sea-weeds. It is more dillicult to account for the origin of the flint. Prof. Bailey found that some seas, especially in the Arctic regions, supplied an enormous quantity of siliceous frustales of the diatoinacete, and spicules of sponges. That such organisms may have been converted into the flint nodules seems very probable, when we remember that many of the nodules have the external conformation of sponges, and show occasionally also the internal structure. Mr. Bowcrbank's microscopic examina tion of flint nodules, seems to lead to the conclusion that all flints arc produced from the siliceous skeletons of organic beings. Chalk, then, seems to have been a deposit in very deep seas, far out of the reach of land-currents, which would certainly have brought with them argillaceous and arenaceous debris.
The C. G. is highly fossiliferous. The remains of plants are abundant in the fresh-. water wealden beds; amongst them have been found fragmentary portions of dicotyle dons. If we except the microscopic diatomacem, which are not unfrequent in the white chalk, vegetable remains are rare in the other members of the group. The various divisions of the animal kingdom are represented in the organic remains of the chalk, if we except the warm-blooded vertebrata, which have hitherto—if they existed—escaped notice. Foraminifera were enormously abundant in the seas, and active in the secre tion of the soluble carbonate of lime, fixing it in their minute shells, which, after their death, as has been shown, formed the principal material of the chalk. In the lower beds, polyzoa have been found in great abundance on the continent. Echinoderms are in immense numbers, and beautifully preserved. Crustacea are occasionally found. Of mollusea, the brachiopoda and cephalopoda are especially abundant, both being pelagic types. Ctenoid and cycloid fishes appear in this group for the first time, though yet in small numbers—the placoids and ganoids being still the predominant forms. Reptiles, though not so numerous as in the former period, were yet far from rare.
For further details of the fossils, See DIATOMACFLE, VENTRICITLITES, FORAMINIFERA,