XI. Flint.--_Animal Remains in siliceous nodules._--So many beautiful specimens of siliceous petrifactions--that is, animal and vegetable remains transmuted into silex or flint--are figured in the subjoined plates, that it may be useful to offer a few remarks on this subject.[112] In many instances the organic remains in chalk-flints are simply incrusted by the silex; such is the state of numerous sponges which are as it were invested by the flint, and have all their pores and tubes filled up by the same material, the original tissue appearing as a brown calcareous substance. In other examples, the sponge has been enveloped in a mass of liquid flint, and has subsequently perished and decomposed; in this manner have been formed those hollow nodules, which on being broken present a cavity containing only a little white powder, or some fragments of silicified sponge; in many instances the cavity is lined with quartz crystals, or mammillated chalcedony. Frequently but part of the zoophyte is permeated by the silex, and the other portion is in the state of a friable calcareous earth imbedded in the chalk. Sponges and other zoophytes often form the nuclei of the flint nodules; the original substance of the organic body being in general silicified, and the most delicate internal structure preserved. Shells, corals, and the minute cases of foraminifera, are often immersed as it were in pure flint, appearing as if preserved in a semi-transparent medium. [Footnote 112: See Wonders of Geology, vol. i. pp. 74-105, for a general view of the process of petrifaction.] But there are innumerable flint nodules in which no traces of spongeous tissue are apparent, and veins, dikes, and sheets of tabular flint, that are in a great measure free from organic remains; containing only such as may be supposed to have become imbedded in a stream of fluid silex that flowed over a sea-bottom. Wood perforated by lithodomi and silicified, is occasionally met with; and fuci or algæ are sometimes found, appearing as if floating in the liquid flint. For the most part, the minute shells in the chalk and flint are filled with amorphous mineral matter; but in many examples, (as I have ascertained by direct experiment,) the soft parts of foraminifera remain in the shell. XII. Foraminifera.--Plate LXII. contains figures of several species belonging to various genera of those minute fossil shells, the discoidal involute forms of which were once considered to belong to the Cephalopoda, and to be related to the Nautilus, Spirula, &c., but which are now grouped in one family, under the name of _Foraminifera_; a term derived from the foramina or perforations with which their shells are traversed, and which have relation to the peculiar organization of the animals.