In another place, Mr. Darwin observes,—"The number of the remains of these large quadrupeds imbedded in the grand estuary deposit which forms the Pampas and covers the granitic rocks of Banda Oriental, must be extraordinarily great. I believe, a straight line drawn in any direction through the Pampas, would cut through some skeleton or bones. Besides those which I found during my short excursions, I heard of many others; and the origin of such names as, 'the stream of the animal,' 'the hill of the giant,' is obvious. At other times, I heard of the marvellous property of certain rivers, which had the power of changing small bones into large; or as some maintained, the bones themselves grew. As far as I am aware, not one of these animals perished, as was formerly supposed, in the marshes or muddy river-beds of the present land, but their bones have been exposed by the streams intersecting the subaqueous deposit, in which they were originally imbedded. We may conclude that the whole area of the Pampas is one wide sepulchre of these extinct gigantic quadrupeds."[111]
[111] Mr. Darwin's Journal, p. 135. The reader interested in these extraordinary fossil remains should visit the British Museum, and the Hunterian Museum of the Royal College of Surgeons in Lincoln's Inn Fields.
[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.
[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.
Since microscopic observations have become so general, thanks to the genius and enthusiasm of Ehrenberg, these fossil bodies have acquired a degree of interest and importance, unsurpassed by more obvious organic remains. Whole mountain chains and extensive tracts of country are now known to be almost entirely composed of the aggregated shells of a few genera of these microzoa.[113] In other deposits their remains are associated with those of Infusoria,[114] (both animal and vegetable,) still more infinitesimal. As much error prevails among collectors as to the real nature of the fossil foraminifera, I am induced to annex the following remarks.[115]
[113] A convenient term to express animal organisms that can only be distinctly examined by the aid of the microscope: strata in a great measure composed of such fossil remains may be distinguished as microzoic deposits.
[114] This term was first employed to denote the various minute forms of animal organization that appear in vegetable infusions; as Rotifers, Monads, Vorticella, &c. But with these, numerous vegetable forms generally appear, as Gaillonella, Bacellaria, Navicula, &c.: these were formerly also regarded as animals, and were consequently comprised under the same general appellation.