GREENSAND, an important member of the cretaceous group of stratified rocks. In Europe it is found in both divisions of these rocks, the upper and lower, the clay called gault being intermediate. The chalk overlies the greensand; and the Wealden clays, where they appear at all, separate it from the next inferior group, the oölite. In the United States, the greensand is not found throughout the range of the cretaceous group around the southern termination of the Alleghanies and thence west. It is indeed little known except on its range through New Jersey. The tract it occupies, commencing at the N. E. on Sandy Hook bay, extends S. on the coast to Shark inlet, giving a width across the Atlantic outcrop of the formation from N. W. to S. E. of about 18 m. Its length is directed S. W. across the state, the tract gradually growing narrower and terminating in a point at Salem, opposite the N. part of Delaware. Its N. line approaches within 1½ m. of the Delaware at Bordentown, and is but little further back from it a few miles below Camden, opposite Philadelphia. The dip of the formation is toward the S. E. at a small angle. On this side its uppermost strata disappear beneath the sands which cover the country; and on the N. W. come up from beneath its lowest beds the clays, well known at Amboy and other points on their range toward the S. W. for their use in pottery and the manufacture of fire brick. A straw-colored limestone, which occasionally appears overlying the greensand on its S. E. margin, calls to mind by its position and the numerous marine fossils it contains the calcareous strata of Europe known as the chalk. The whole thickness of the strata known as greensand is about 100 ft., but one principal bed is recognized among the other strata of sand and clays and intermixed greensand, which is about 30 ft. thick. This is in great part, sometimes wholly, made up of small round dark granules; several are often united in one, and a quantity of them moistened may sometimes be kneaded like clay. The grains are commonly of deep green color, sometimes bluish, and sometimes a dark chocolate; but whatever their external color may be, they are all bright green when well washed, and especially when crushed. Clay and white silicious sand are commonly intermixed in variable proportions with the greensand. In some places fossil shells and other marine organic remains abound in the greensand, being grouped together in layers a few feet in thickness. The species are numerous and often beautifully preserved. This is especially the case with those found in the overlying yellowish limestone; all are extinct. Of 60 shells collected by Lyell, 5 proved to be identical with European species, viz.: ostrea larva, O. vesicularis, gryphæa costata, pecten quinquecostatus, belemnites mucronatus. Prof. Forbes regarded 15 of the 60 “as good geographical representatives of well known cretaceous fossils of Europe.” Besides these organic remains are found teeth and vertebræ of sharks and some other fishes, also teeth and other vestiges of crocodiles and several other saurians, some of gigantic size, one of the largest of which, the hadrosaurus Faulkii, has been restored from a few bones by Prof. B. Waterhouse Hawkins, and is now deposited in the museum of the Philadelphia academy of science. Remains of several crustaceous animals, as crabs, are also met with, and finely preserved specimens of various species of the echinodermata, and of zoöphytes, sponges, &c. The shells which most abound in the greensand, occasionally making up the principal portion of the layers in which they occur, are gryphæas, terebratulas, ostreas, belemnites, and the exogyra costata, the last named a very common and large bivalve peculiar to the cretaceous group.—The greensand is of importance for its fertilizing property; and this is found to be derived, not from the calcareous nature of its organic remains, but from the green grains which commonly make up the greater portion of its beds. These, as they are found in New Jersey, when separated from adhering sand and clay, present a composition varying only within a limited range, and not differing from that of the greensand near Havre, France, as determined by Berthier. But according to the analysis of Dr. Turner, the same mineral substance of Kent, England, is deficient in the very element, potash, to which its valuable qualities in this country are essentially owing. Some of the same material also met with in Marshfield, Duxbury, and Gay Head, Mass., resembles the English in this particular. The mean of four analyses of New Jersey greensand, made by John C. Smock and E. H. Bogardus in 1865-'8, and the result of the examination of foreign specimens, are given in the following table:
| CONSTITUENTS. | N. J. | France. | Gay Head. | England. | |||||||||
| Silica |
|
50.00 | 56.70 | 48.50 | |||||||||
| Potash | 10.90 | .... | .... | ||||||||||
| Lime | .... | 1.62 | .... | ||||||||||
| Alumina | 7.00 | 13.32 | 17.00 | ||||||||||
| Protoxide of iron | 21.00 | 20.10 | 22.00 | ||||||||||
| Peroxide of iron | |||||||||||||
| Water | 11.00 | 7.00 | 7.00 | ||||||||||
| Magnesia | .... | 1.18 | 3.80 | ||||||||||
In New Jersey the greensand (there called marl) is dug from pits during the winter, and brought out upon the fields, where it is spread to be ploughed in. The effect is experienced with the first crop, and continues for several years.—The investigations of Ehrenberg first showed that many of the greensand grains are casts of the microscopic shells of polythalamia (the many-chambered) and of other organic bodies. The shells themselves had disappeared; but the internal form of their cavities was retained in the more durable silicate of iron, which took the place of the animal bodies as these decayed, and preserved their shapes. Even the very finest canals of the cell walls, and all their connecting tubes, are thus petrified and separately exhibited. Many of the grains which cannot be recognized as of this origin still suggest some connection with animal bodies by their forms being sometimes lobed and again presenting the appearance of coprolites. Prof. Bailey by his experiments confirmed the conclusions of Ehrenberg, and, extending his investigations to cretaceous rocks from Alabama and W. Texas, found attached to them grains of greensand exhibiting the same phenomena. From specimens of marl and limestone of the eocene of the southern states he also succeeded in bringing to light similar grains of the same character by dissolving away with dilute acid the calcareous matters. One of his specimens was brought up in sinking the artesian well at Charleston from the depth of 140 ft. The soundings of the coast survey brought up from the depths of the ocean, in the Gulf stream and the gulf of Mexico, something resembling greensand. Count Pourtalès reports one sounding as of this character obtained in lat. 31° 32', lon. 79° 35', at the depth of 150 fathoms. This, as well as the others referred to, were examined by Prof. Bailey, who found them to be greensand, and that this is often in the form of well defined casts of polythalamia, minute mollusks, and branching tubuli. The material he found to be the same as that of the fossil casts; but the chief part of the soundings he found consisted of perfectly preserved shells of the same species, which retained their brilliant colors, and gave evidence by treatment with acid that the soft parts were still present, thus proving the recent existence of the animals. Hence it appears that in some deep seas the production of greensand is still going on, and formations of this obscure material are there growing up by the same agencies which elaborated those of ancient geological periods.