The assertion that the chalk every where consists almost wholly of organic bodies must likewise be accepted with some limitation. The assiduous observer who searches for hours chalk and flint carefully prepared, and with the aid of an excellent microscope, though he will meet with immense numbers of organisms, will often find a great proportion of atoms without traces of structure. Neither is there much variety in the easily recognizable forms of the English chalk (I write from my own limited experience); many of the species described by M. Ehrenberg, and others, are few and far between; and I have not detected a single example of diatomaceæ. The student therefore must not be discouraged, if, after perusing the glowing accounts of the discoveries of M. Ehrenberg, he should not be more successful than myself. It must, however, be borne in mind, that as the fossil remains of the larger animals and plants are commonly associated together in particular localities, while in similar rocks in other districts they are altogether wanting; in like manner, some strata of the same series may be made up of organic bodies, while others are destitute of them. In fact, such is the case with our English Chalk: some layers in the cliffs at Dover are literally an aggregation of foraminifera and corals, while other beds have but few vestiges of organic remains. [Sidenote: CHALK FORAMINIFERA.] [Illustration: Lign. 115. Chalk-dust; chiefly composed of Foraminifera; _highly magnified._ (_Seen by reflected light._) _a, a._ Rotaliæ. _b._ Textulariæ.] Foraminifera of the Chalk and Flint. _Lign. 115._--If a few grains of soft white chalk from Gravesend or Dover be examined under a high power (1/4 inch object-glass of Ross), groups of foraminifera will be perceived, chiefly of Rotaliæ, Rosalinæ, and Textulariæ, as shown in _Lign. 115_. If some of the powder be immersed in Canada balsam, (as directed in the instructions at the close of this chapter for preparing chalk for microscopical examination,) the outline of the shells, and the cavities of, the cells, will be apparent; as in the sketches _Lign. 114_, _ante_ p. 351. If a chip or slice of flint, rendered transparent by immersion in oil of turpentine or Canada balsam, be viewed first with a low power, (1 inch object-glass,) to discover a good specimen, and afterwards under a high magnifier, (a 1/4 or an 1/8 object-glass,) the form of the shell and of the cells will be distinctly seen, as in _Lign. 116_. In this beautiful fossil Rotalia, the segments are as sharply defined as in a recent example: and one of the cells (_a_) is seen to be lined with quartz crystals. [Illustration: Lign. 116. Section of a Rotalia in flint. (_Seen by transmitted light, and showing the chambers partially filled with mineral matter._) _a._ A cell lined with spar. (× 250 _diameters_.)]