NATURE STUDY IN SCHOOLS. 105 



LIMEST ONE. 



BY 



Wm. D. McPherson. 



We now come to the second kind of rock, limestone ; while we find 

 granite composed of hard minerals of two, three, or four kinds, we find that 

 pure limestone is composed of a single kind, namely carbonate of lime and this 

 is always quite soft. While it may be true that some silica or quartz, which 

 is found in granite may have been taken up by plants and re- deposited again r 

 we cannot consider granite as being of organic origin . 



On the other hand, it is probable that all of the limestone in the world 

 has at one time or another passed through the bodies of animals. Thus it is 

 of true organic origin, and is very often the true store house of fossils. 



Although pure limestone is thus made of one single kind of mineral 

 substance, there are various kinds of limestones. These may be divided 

 into two groups, sedimentary, which are the more recent, and chrystaline, 

 which are the more ancient. 



We speak of recent limestone, and while it must be understood that 

 the term is comparative, and that some fossiliferous limestones were formed 

 long ages ago, it is also true that limestone is being formed to-day. If we 

 gather ooze from deep ocean beds, we will find that it is made up of nu- 

 merous minute shells, which are called foraminifera, and the animals which 

 formed them are living to-day. If we let a quantity of this ooze dry, it 

 forms a kind of soft, gray, chalky rock, which is wholly limestone or chalk. 



In fact, the chalk beds of Europe were all principally formed of the 

 minute shells of these small foraminifera that lived in ocean beds of ages 

 passed, but which have since dried up. Chalk, then, is the softest kind of lime- 

 stone. Coquina is another which is composed of shells, corals and their frag- 

 ments, ground and broken into pieces of varying size by the action of the 

 waves. Florida is made up largely by an underlying state of coquina limestone. 



Aeolian limestone is made up of coral, shells etc. These are broken at 

 first into coarse pieces by the action of the waves, especially during hurricanes 

 and afterwards ground finer by being dashed back and forth repeatedly on the 

 ocean beaches by the powerful waves. When these pieces get fine enough, 

 to form a sand, they are blown by the winds into the interior of the land, 

 and accumulating in quantities, become cemented together through moisture 

 and pressure and thus form aeolian limestone, that is, limestone made by 

 Aeoleus, the god of the winds. The Bahama Islands are largely made of 

 this kind of limestone. 



