THE STUDY OF THE ROCKS 



microscope. We shall find that the substance we 

 examine consists of sand containing a goodly number 

 of plates of mica and there may also be a number 

 of sponge spicules. We must bear in mind when 

 we are examining the remnants of limestone after 

 treatment with acid, that shells are largely composed 

 of calcium carbonate, so it is useless to look for 

 any shell remains, for the acid will have dissolved 

 them. 



Calcium carbonate, in a nearly pure state, will 

 and does form rock-like structures; we are most of 

 us familiar with stalactites which are formed on the 

 roofs of caves. These structures are usually com- 

 posed of calcium carbonate though sometimes, 

 notably in some of the Derbyshire caves, Barium 

 takes the place of Calcium. Should we have the 

 chance of examining a section of a stalactite we 

 should certainly do so. We can see the rings which 

 are formed, as layer after layer of calcium carbonate 

 is deposited, in fact the section of a stalactite bears 

 a striking resemblance to a stem of a tree and has, 

 before now, been mistaken for a fossil stem. 



If the study of rocks appeals to us we should 

 make a point of examining all the specimens we 

 can lay hands upon. Many quite common speci- 

 mens may easily be obtained; rock-salt, for ex- 

 ample, though in itself not of great interest as an 

 object for the microscope, will readily dissolve in 

 water, leaving behind an insoluble residue of iron 

 which is well worth examination. The majority of 

 rocks, however, are not affected by water and but 



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