(Magnified nearly 100 diameters.)]

The examination of a transparent slice gives a good notion of the manner in which the components of the chalk are arranged, and of their relative proportions.  But, by rubbing up some chalk with a brush in water and then pouring off the milky fluid, so as to obtain sediments of different degrees of fineness, the granules and the minute rounded bodies may be pretty well separated from one another, and submitted to microscopic examination, either as opaque or as transparent objects.  By combining the views obtained in these various methods, each of the rounded bodies may be proved to be a beautifully-constructed calcareous fabric, made up of a number of chambers, communicating freely with one another.  The chambered bodies are of various forms.  One of the commonest is something like a badly-grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together.  It is called Globigerina, and some specimens of chalk consist of little else than Globigerinae and granules.

[Illustration:  GLOBIGERINA.]

Let us fix our attention upon the Globigerina.  It is the spoor of the game we are tracking.  If we can learn what it is and what are the conditions of its existence, we shall see our way to the origin and past history of the chalk.

A suggestion which may naturally enough present itself is, that these curious bodies are the result of some process of aggregation which has taken place in the carbonate of lime; that, just as in winter, the rime on our windows simulates the most delicate and elegantly arborescent foliage—­proving that the mere mineral matter may, under certain conditions, assume the outward form of organic bodies—­so this mineral substance, carbonate of lime, hidden away in the bowels of the earth, has taken the shape of these chambered bodies.  I am not raising a merely fanciful and unreal objection.  Very learned men, in former days, have even entertained the notion that all the formed things found in rocks are of this nature; and if no such conception is at present held to be admissible, it is because long and varied experience has now shown that mineral matter never does assume the form and structure we find in fossils.  If anyone were to try to persuade you that an oyster-shell (which is also chiefly composed of carbonate of lime) had crystallized out of sea-water, I suppose you would laugh at the absurdity.  Your laughter would be justified by the fact that all experience tends to show that oyster-shells are formed by the agency of oysters, and in no other way.  And if there were no better reasons, we should be justified, on like grounds, in believing that Globigerina is not the product of anything but vital activity.

Happily, however, better evidence in proof of the organic nature of the Globigerinae than that of analogy is forthcoming.  It so happens that calcareous skeletons, exactly similar to the Globigerinae of the chalk, are being formed, at the present moment, by minute living creatures, which flourish in multitudes, literally more numerous than the sands of the sea-shore, over a large extent of that part of the earth’s surface which is covered by the ocean.