The nodular flint has collected round the sponge, while the sponge itself has often disappeared. G. J. Hinde[21] has shown how readily the spicules of siliceous sponges go into solution. Even at the bottom of existing seas they become rounded at the ends, while their canals become enlarged. In some fossil instances, they are replaced by calcite. W. J. Sollas[22], emphasising this point, remarks that "it may be taken as an almost invariable rule that the replacement of organic silica by calcite is always accompanied by a subsequent deposition of the silica in some form or other." This subsequent deposition is frequently at the expense of calcite in some other part of the rock. The solid flint is a replacement of the limestone in which it occurs. The pocket-lens will often show traces of sponge-spicules, as dull little rods, in the translucent substance of a flint. But the microscope shows that the mass of the flint has the structure of the limestone in which it lies. The foraminifera and other small structural features of the original rock are perfectly preserved in chalcedonic (that is, minutely crystalline) silica. Larger fossils, such as thick molluscan shells and the tests of sea-urchins, may escape alteration, while the chalk mud, the original ooze, with which they are infilled has become completely silicified. This explains the internal moulds of fossils in brown oxidised flint that are found in gravel-pits on the surface of the Chalk, and also the tubular hollows, representing stems of crinoids, that often occur in flint from the Carboniferous Limestone. In the latter case, the fossil remained calcareous while the ground became silicified, and the fossil was removed by subsequent solution. Where great thicknesses of strata, as may happen in the Carboniferous Limestone, have become thus silicified, it may be presumed that siliceous skeletons were unusually abundant in the mass. But, as L. Cayeux[23] observes, such skeletons may be in one case entirely removed, and in another represented by massive flints; in yet another case, the silica may remain disseminated through the rock. The irregularity of its segregation is shown by the growth of flints in branching or hook-like forms, running from one bed to another in a limestone. Oolitic limestones and the skeletons of corals, both having been originally made of aragonite, are often replaced by flint, forming conclusive instances, appreciable by the naked eye, of the secondary origin of this form of silica. Traces of diatoms are comparatively rare, though they probably contributed to the silicification of the freshwater Calcaire de la Brie of the Paris basin. Radiolaria, however, have now been well recognised as flint-formers, even in dark "cherts" of Silurian age. Radiolarian cherts have been taken as an indication that the beds in which they occur were formed in oceanic depths.