I ON A PIECE OF CHALK 7 



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 com- 

 ponents 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 some- 

 thing like a badly -grown raspberry, being formed 

 of a number of nearly globular chambers of 

 different sizes congregated together. It is called 

 Gldbigerina, and some specimens of chalk consist 

 of little else than G-lobigerince 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. 



