ON A PIECE OF CHALK 57 



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 appear- 

 ance when placed under the microscope. The general mass 5 

 of it is made up of very minute granules ; but embedded in 

 this matrix, are innum^gj^^OTO V lUU^smaller and some 

 larger, but, on a rougS^e^^e] not 4ft$rakn a hundredth 

 of an inch in dj^m^bf, having a well-defined shape and 

 structure. A cuoic inch of some .specHTtens i|f chalk may 10 

 contain hundred^ ofV&t)Usand's of these bodies, compacted 

 together with mcaxujabl^ rnillioj^ $%hj^&&nules. 



The examination mr-frrii fflnpn rffnt'-nffrr i'"p" a good notion 

 of the manner in which the components of the chalk are ar- 

 ranged, and of their relative proportions. But, by rubbing 15 

 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 2 o 

 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 25 

 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 Globigerince 

 and granules. 30 



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. 



