ix.] ON A PIECE OF CHALK. 155 



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 ex 

 amination, 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-con 

 structed 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 Globigcrincc 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 may, under certain conditions, assume 



