The upper marine sands (1. _b_), consist chiefly of micaceous and quartzose sands, 80 feet thick. As they succeed throughout an extensive area deposit of a purely freshwater origin (2 _a_.), they appear to mark a subsidence of the subjacent soil, whether it had formed the bottom of an estuary or a lake. The sea, which afterwards took possession of the same space, was inhabited by testacea, almost all of them differing from those found in the lower formations (2. _b_ and 2. _c_) and equally or still more distinct from the Miocene Faluns of subsequent date. One of these upper Eocene strata in the neighbourhood of Paris has been called the oyster bed, "couche a _Ostrea cyathula_, Lamk.," which is spread over a remarkably wide area. From the manner in which the oysters lie, it is inferred that they did not grow on the spot, but that some current swept them away from a bed of oysters formed in some other part of the bay. The strata of sand which immediately repose on the oyster-bed are quite destitute of organic remains; and nothing is more common in the Paris basin, and in other formations, than alternations of shelly beds with others entirely devoid of them. The temporary extinction and renewal of animal life at successive periods have been rashly inferred from such phenomena, which may nevertheless be explained, as M. Prevost justly remarks, without appealing to any such extraordinary revolutions in the state of the animate creation. A current one day scoops out a channel in a bed of shelly sand and mud, and the next day, by a slight alteration of its course, ceases to prey upon the same bank. It may then become charged with sand unmixed with shells, derived from some dune, or brought down by a river. In the course of ages an indefinite number of transitions from shelly strata to those without shells may thus be caused. Besides these oysters, M. Deshayes has described 29 species of shells, in his work (Coquilles fossiles de Paris), as belonging to this formation, all save one regarded by him as differing from fossils of the calcaire grossier. Since that time the railway cuttings near Etampes have enabled M. Hebert to raise the number to 90. I have myself collected fossils in that district, where the shells are very entire, and detachable from the yellow sandy matrix. M. Hebert first pointed out that most of them agree specifically with those of Kleyn Spauwen, Boom, and other localities of Limburg in Flanders, where they have been studied by MM. Nyst and De Koninck.[176-A]