In several glass test-tubes, (the more the greater chance of success,) put a few grains of chalk powder: pour the tube half full of diluted hydrochloric (muriatic) acid--about ten parts water to one of acid--agitate, and set the mixture by: when all action has ceased add one or two drops of undiluted acid to each tube, and repeat the process at due intervals till all the calcareous matter is dissolved. Pour off the fluid, substitute distilled water, agitate, and then let the sediment subside. The residue will consist of atoms of quartz and other insoluble mineral matter, and animal tissue, if there be any. Then, with a camel-hair pencil, place a small portion of the sediment on a glass slide, and when dry cover it with Canada balsam, and treat it as above directed. Among a dozen slides thus mounted, there will probably be two or more good examples of the body, or detached membranous segments of Rotaliæ or Textulariæ, like those figured in _Lign. 118_. Calcareous Sandstones and Marls.--These substances may be examined by the same process; but if of loose texture, Dr. Bailey recommends that some of the sandy powder should be spread very thinly on a plate of glass, with or without water, and by the aid of a lens of moderate power the roundish grains should be selected and picked up with fine forceps, or the point of a needle, and transferred to another piece of glass, having on one spot a thin coat of Canada balsam. This should be gently heated over a spirit lamp, when the balsam will penetrate the grains, and render them transparent; by this process the minutest shells, &c. may be detected. For a hasty exploration, the dust may be rendered sufficiently transparent by a drop or two of oil of turpentine. Sandy calcareous marls may be examined by diffusing a few grains in water in a wine-glass, the lighter portions will be suspended in the fluid, and may be placed on glass, and when dry prepared with Canada balsam in the usual manner. [Sidenote: MICROSCOPICAL EXAMINATION OF FLINT.] Flint.--Flint, and other siliceous stones, require to be chipped into very thin fragments, and immersed in oil of turpentine. A clear, translucent flint should be selected, from which fragments may be shattered off by smart blows of a hammer, over a sheet of white paper: the most transparent flakes are to be selected, 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, mount them in Canada balsam. It is hazardous to entrust such fossils to the lapidaries; an interesting group of twenty spiniferites 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. CHAPTER XI. FOSSIL TESTACEOUS MOLLUSCA, OR SHELLS. [Illustration: Lign. 120. Fossil Oyster, from the Chalk. _Kemptown, Brighton._]