fossils.*

In the United States, a range of mountains near Suggsville, three hundred feet high, is entirely composed of one species of nummulite.†

A large proportion of the sand of the Lybian desert consists of microscopic fossil remains. Below the chalk formation some deposits also abound in animalculites. But in the Wealden limestones, sandstones, and clays, I have not succeeded in detecting any trace of infusoria, although, from the abundance of several species of the minute freshwater crustaceans, called Cypris (Wond. p. 380.), it might be expected that the carapaces of Bacillariæ, Naviculæ, &c. on which the living Cyprides feed, would occur in immense quantities. Messrs. Reade, White, Deane, H. Lee, and other observers, have obliged me by repeated examinations of the Wealden rocks, but hitherto without success. I have inspected the Sussex and Purbeck marbles by every method, but have detected nothing except fragments of bones, vegetable matter, and cases of Cyprides, broken or entire, with which the cavities of the shells (paludina) composing those limestones, are literally crammed. In the fresh-water beds

* An interesting fact was mentioned to me by a friend who lately descended the Nile; namely, that the nummulite limestone rocks are in some places washed down and disintegrated, and the loose nummulites re-deposited in the recent detritus, or mud of the river.

+ See Dr. Morton. Cret. Form. of North America.

which overlie the petrified forest of Portland, no infusoria have been discovered. The Stonesfield slate, and the mountain limestone of Derbyshire, when sliced and polished, exhibit numerous microscopic Polythalamia, and other shells.

INSTRUCTIONS FOR THE MICROSCOPICAL EXAMINATION OF CHALK, AND OTHER LIMESTONES; FLINT, ETC.

The following method is that recommended by M. Ehrenberg. Place a drop of water upon a plate of thin glass, and put into it as much scraped chalk as will cover the fine point of a knife, spreading it out, and leaving it to rest a few seconds; then withdraw the finest particles which are suspended in the water, together with most of the liquid, and let the remainder become perfectly dry. Cover this dried spot of chalk with Canada balsam (the turpentine of Abies balsamea), and hold the plate of glass over the flame of a lamp, until the balsam becomes slightly fluid, without froth or air-bubbles; it should be maintained in this position (the glass being kept as hot as the finger will bear) for a few minutes, until the balsam is found to have thoroughly permeated the substance to be examined. It is preferable to place a piece of very thin glass upon the balsam, and gently press it down, and allow it to remain. (Glass for this purpose may be obtained of Mr. Drake,

Jermyn-street, St. James's, London. The thinnest

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is of an inch thick, and is 3s. 6d.

100

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per ounce; the thicker kind is so of an inch, and is cheaper.) The best flatted crown-glass should be used for placing the chalk or other objects on. It is convenient to have the slips of glass of one size, or the specimens will require different boxes for their reception; three inches by one inch, is that usually employed. These objects require to be viewed with a power magnifying three hundred times linear, that is, in diameter; and if the process has been properly conducted, it will be seen that the chalk is chiefly composed of well-preserved organisms. In these preparations all the cells of the Polythalamia appear at first black, with a white central spot, which is caused by the air contained in those cavities, for air-bubbles always appear under water as black annular bodies; but, by degrees, the balsam penetrates into all the single cells, the black rings of the air vesicles disappear, and the structure of the originals is beautifully displayed.*

CALCAREOUS SANDSTONE AND MARL, may be examined by the same process; but if it be of loose texture, Dr. Bailey spreads some of the sandy powder very thinly on a plate of glass, with or

* Ann. Nat. Hist. June, 1841. From a masterly abstract of M. Ehrenberg's Memoir on the Microscopical Structure of Chalk, &c. by Mr. Weaver.

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without water, and by the aid of a lens of moderate power, selects and picks up, with fine forceps, or the point of a needle, the roundish grains, and transfers them to another piece of glass, having on one spot a thin coat of Canada balsam. This should be gently heated over a spirit lamp, and the balsam will penetrate the grains, and render them transparent; by this process the minute Polythalamia, &c. may be detected. For a hasty exploration, the dust may be rendered transparent by a drop or two of oil of turpentine.

In sandy calcareous marls, as some of the infusorial earths of Virginia, the same accurate observer directs that a few grains should be diffused in water in a watch-glass, the lighter portion will be thus suspended in the fluid, which should be dropped on glass, and when dry prepared with Canada balsam, as above.

Flint, and other siliceous stones, require no preparation, but may be examined in the manner already pointed out in the description of the fossil Xanthidia, by chipping off very thin fragments, and immersing them in oil of turpentine. A clear, translucent flint should be selected, from which the slices should be chipped by smart blows of a hammer, over a sheet of white paper: the most transparent flakes are to be preferred, and these should be put in oil of turpentine, in a wide-mouth glass bottle. Take out the pieces for examination with forceps, and inspect them as transparent objects,

by transmitted light. When good specimens are discovered, they should be mounted in Canada balsam. It is hazardous to entrust these fossils to the lapidaries; the interesting group of twenty Xanthidia, presented to me by Mr. Lee, was reduced to ten, by one of our best workmen, in whose hands it was placed for polishing, with the view of rendering it more transparent.