6 ON A PIECE OF CHALK I 



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 con- 

 trary, 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 sub- 

 stance, 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 in- 

 numerable 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 



