This chalk is of a white colour, very light and porous; under the higher powers of the microscope, it appears to be made up of organic forms, as "Foraminifera," and portions of various kinds of shells, crushed and broken into minute fragments (fig. 27). Dr. Carpenter, in his work on the microscope, says: "Many parts of it (the chalk) consist in a great measure of the minuter parts of the smaller kinds of Foraminifera, whose shells are imbedded in a mass of apparently amorphous particles, many of which nevertheless present indications of being the worn fragments of similar shells, or of larger calcareous organisms. In the chalk of some localities, Foraminifera constitute the principal part of the minute organisms which can be recognised with the microscope; in other instances the disintegrated prisms of Pinnæ, or other large shells of the like structure (as Inoceramus), constitute the great bulk." The fossil remains in the chalk are very numerous and are all of a marine character, the ammonites (fig. 28), belemnites, and other cephalopods, were very prevalent, as were the various Echinodermata, as the _Hemicidarus intermedia_ (fig. 29), together with numerous univalve and bivalve mollusca, various crustacea, fish and reptiles. There was some considerable wonderment a few years ago expressed at the skeletons of men being found in the chalk at Guadaloupe; but it has been ascertained that this chalk is a modern formation, being produced by the sea washing and disintegrating the adjacent coral reefs, and depositing a fine white sediment of broken coral on the shore which can hardly be distinguished from ancient chalk; the same process is taking place at the Bermudas and other islands of the West Indies. [Illustration: FIG. 29.--ECHINUS (Hemicidarus intermedia, Chalk).] In many places the chalk strata contain single lines of flints, running for miles parallel to the layers of chalk; these flints consist of almost pure silica, and it has been, matter of wonder how they got there, but on considering how slowly the deposition of chalk must have taken place, from the formation and death of millions of minute creatures, and that it was once the bottom of a deep sea, the disposition of the flints in lines would be accounted for, supposing them to have been formed on that sea-bottom, and the source from which they have been derived is doubtless the petrifaction of sponges, madrapores, &c., there formed. Dr. Carpenter (in the work before referred to) says: "It may be stated, as a fact beyond all question, that nodular flint and other analogous concretions (such as agates) may generally be considered as fossilised sponges or alcyonian zoophytes, since not only are their external forms and their superficial markings often highly characteristic of those organisms, but when sections of them are made sufficiently thin to be transparent, a spongy texture may be most distinctly recognised in their interior." [Illustration: FIG. 30.--TERTIARY FORMATION.]