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. 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.