It is desirable for us to start from the knowledge of this fact, though it may not seem to help us very far towards what we seek.  For carbonate of lime is a widely-spread substance, and is met with under very various conditions.  All sorts of limestones are composed of more or less pure carbonate of lime.  The crust which is often deposited by waters which have drained through limestone rocks, in the form of what are called stalagmites and stalactites, is carbonate of lime.  Or, to take a more familiar example, the fur on the inside of a tea-kettle is carbonate of lime; and, for anything chemistry tells us to the contrary, the chalk might be a kind of gigantic fur upon the bottom of the earth-kettle, which is kept pretty hot below.

Let us try another method of making the chalk tell us its own history.  To the unassisted eye chalk looks simply like a very loose and open kind of stone.  But it is possible to grind a slice 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 Globigerinoe 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.