e of chalk down so thin that
you can see through it--until it is thin enough, in fact, to be examined
with any magnifying power that may be thought desirable. A thin slice of
the fur of a kettle might be made in the same way. If it were examined
microscopically, it would show itself to be a more or less distinctly
laminated mineral substance, and nothing more.

But the slice of chalk presents a totally different appearance when
placed under the microscope. The general mass of it is made up of very
minute granules; but imbedded in this matrix, are innumerable bodies,
some smaller and some larger, but, on a rough average, not more than a
hundredth of an inch in diameter, having a well-defined shape and
structure. A cubic inch of some specimens of chalk may contain hundreds
of thousands of these bodies, compacted together with incalculable
millions of the granules.

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.

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 water