In the case of recent and fossil Foraminifers, these--when not so little mineralized that their chambers are empty, or only partially filled, which is sometimes the case even with Eocene Nummulites and Cretaceous forms of smaller size,--are very frequently filled solid with calcareous matter, and as Dr. Carpenter well remarks, even well preserved Tertiary Nummulites in this state often fail greatly in showing their structures, though in the same condition they occasionally show these in great perfection. Among the finest I have seen are specimens from the Mount of Olives (fig. 19), and Dr. Carpenter mentions as equally good those of the London clay of Bracklesham. But in no condition do modern Foraminifera or those of the Tertiary and Mesozoic rocks appear in greater perfection than when filled with the hydrous silicate of iron and potash called glauconite, and which gives by the abundance of its little bottle-green concretions the name of "green-sand" to formations of this age both in Europe and America. In some beds of green-sand every grain seems to have been moulded into the interior of a microscopic shell, and has retained its form after the frail envelope has been removed. In some cases the glauconite has not only filled the chambers but has penetrated the fine tubulation, and when the shell is removed, either naturally or by the action of an acid, these project in minute needles or bundles of threads from the surface of the cast. It is in the warmer seas, and especially in the bed of the Ægean and of the Gulf Stream, that such specimens are now most usually found. If we ask why this mineral glauconite should be associated with Foraminiferal shells, the answer is that they are both products of one kind of locality. The same sea bottoms in which Foraminifera most abound are also those in which for some unknown chemical reason glauconite is deposited. Hence no doubt the association of this mineral with the great Foraminiferal formation of the chalk. It is indeed by no means unlikely that the selection by these creatures of the pure carbonate of lime from the sea-water or its minute plants, may be the means of setting free the silica, iron, and potash, in a state suitable for their combination. Similar silicates are found associated with marine limestones, as far back as the Silurian age; and Dr. Sterry Hunt, than whom no one can be a better authority on chemical geology, has argued on chemical grounds that the occurrence of serpentine with the remains of Eozoon is an association of the same character.