The egg-like particles vary considerably in size, being in some cases almost invisible to the naked eye, whilst in others they are nearly as large as peas; this last form of Oolite has received the name of Pisolite, differing, however, from the true Oolites only in the relative size of the globular concretions. Bath stone, Portland stone, and the slate of Stonesfield, near Oxford, are all examples of Oolite. In Fig. 52, A, is represented

A, Portion of oolite termed Roe-stone or Pisolite. B, Granules from Britannia rock, magnified 40 diameters. c, Granule of Pisolite magnified 12 diameters. D, Granule of Oolite from Germany, magnified 20 diameters.

a portion of that form of Oolite termed Pisolite of its natural size; the granules are th of an inch in diameter, one of them, shown in section at c, is magnified 12 diameters, and the concentric lamina of which it is composed are well displayed.

In Germany there is an Oolite in which the granules are nearly as large as they are in the Pisolite, but the

concentric laminated arrangement, as shown at D, and the presence of a central nucleus, are more strongly marked; the rock supporting the Britannia bridge is a firm Oolite, in which the granules are remarkably small, those represented by B being magnified 40 diameters. The specimens just described are all very compact, the granules being firmly cemented together by the calcareous material forming the matrix; it sometimes happens, however, in oolitic districts, that the granules are separated from the matrix, and the soil will be seen to be in a great measure made up of them, this is especially the case in the neighbourhood of Bath; the soil of High Barrow Hill, I found to be so rich in oolitic granules, that when turned up by the plough, it appeared as if thickly sown with minute yellow seeds.

Before I leave the subject of chalk and flint, I must say a few words on certain remains of organized beings which have been met with in both the above substances; they were first discovered in flint by the Rev. J. B. Reade, in 1838, who published an account of them in "The Annals of Natural History" for that year, and named them Xanthidia, from their resemblance, both in form and colour, to some recent animalcules described by Ehrenberg under the genus Xanthidium. species were obtained by the Rev. J. B. Reade, which, with a number of new ones, were subsequently described by Mr. Henry Hopley White in the first volume of "The Transactions of the Microscopical Society." They consist for the most part of a hollow spherical body,

Many

from which a number of spines or arms radiate, some of these arms terminating in a point, others more commonly dividing into two or three branches, which, like those of the spicula of certain sponges, are expanded or hamular; the spines, like the body, are hollow, and in the species termed by Mr. White, tubiferum, the extremity of each spine is expanded like a sucking disc.

Xanthidia of the form represented by 1 in Fig. 20, were discovered alive in the neighbourhood of West Point, New York, by Professor Bailey, and were sent to this country in a living state; living Xanthidia were subsequently discovered in many pools in the vicinity of London. From their green colour and their mode of increase by conjugation, which has been so well described and figured by Mr. Ralfs, there is every reason to believe that they belong to the Desmidieæ, and in proof of this it can be shown that their skeleton is composed of horn, and not of silica, as was at first imagined.

In Xanthidium spinosum, Fig. 20, G, the arms, or tentacula, as they have been termed by Mr. White, are sharp pointed, but in X. ramosum, shown at K, they are much branched at their free extremities, while in Xanthidium recurvatum, represented by H, the extremities of the spines, generally from four to six in number, are all bent back like so many hooks. The Xanthidia, when first discovered in flint, created a great sensation amongst microscopists; and such was the anxiety to obtain specimens, that from first to

last, several tons of flints were broken up, or cut, in

order to find them. Fossil Xanthidia are not confined

A, B, C, D, Xanthidia discovered in the mud of the Thames at Greenhithe. E, F, Xanthidia from the chalk.

to flints, they have been obtained by Mr. Deane from the chalk between Folkstone and Dover, in which there were no flint nodules; two well-marked species are represented by E and F in Fig. 53. In one piece of a greyish kind of chalk from this locality, Mr. Deane discovered no less than six species by treating the chalk with hydrochloric acid; they were accompanied by Polythalamia and the remains of other organized bodies named Rotalia, as shown at D in Fig. 48, and he was enabled to prove clearly that the Xanthidia possess a horny skeleton.

A short time after the discovery of Xanthidia in the chalk, recent specimens were obtained by Mr. J. T. Norman from some mud adhering to one of the piles supporting the pier at Greenhithe, on the Thames, a

neighbourhood abounding in chalk. The skeleton of each is of a brown colour precisely like horn, and the arms are generally more or less bent.

An account of these Xanthidia is given in the second volume of "The Transactions of the Microscopical Society," by Mr. S. J. Wilkinson, and from the illustrations accompanying the paper, the four species represented by A B C D, in Fig. 53, have been copied. The slides containing the identical specimens, have been presented to me by Mr. Norman, and you will have an opportunity of noticing a large opening communicating with the interior in almost every individual. Each slide contains numerous spicula of sponge, of the bi-acuate form, and several species of Navicula; the mud was treated both with nitric and sulphuric acids, yet the skeletons of the Xanthidia were not destroyed. Mr. Norman has been very fortunate in finding Xanthidia in flints, and a slice of a small pebble picked up in the street at Islington, he has found to contain no less than thirty-two specimens of fossil Xanthidia.

VOL. II.

H