412 Introduction to the Study of Science 



strata, it dissolves and carries away greater amounts of cal- 

 cium carbonate (page 419). At the surface of the earth the 

 carbon dioxid escapes rapidly, causing bubbling or effervescence, 

 and the limestone is deposited in great abundance. 



What is the origin of the limestone ? Limestones comprise 

 limerock, marble, and chalk. They are composed largely of 

 carbonate of lime, though they contain many other substances 

 which give them different textures and degrees of hardness and 

 color. Some lime formations have already been found to be 

 chemical in origin, i.e. the stalactites and deposits of carbonated 

 springs. 



The great bulk of limestone is, however, of organic origin. 

 The carbonate of lime is made up of the mineral shell substance 

 of myriads of generations of organisms that lived and died in 

 the ocean. The shells of these animals sank to the bottom and 

 there formed in course of time vast quantities of shell substance 

 and fine material called ooze. The accumulated masses of 

 both were later covered, because of tremendous earth changes, 

 with deposits of other materials. The organic remains, lime 

 carbonate and silica (page 424), were cemented together, sub- 

 jected to pressure, and in time formed into rock. 



Near the coasts of bodies of water the lime and silica were 

 mixed with other substances, as clay and sand. Some of the 

 rock resulting from this mixture is intermediate between sand- 

 rock and limerock, and is classed as impure limerock. 



Chalk. The lime carbonate deposits in fairly deep seas 

 formed chalk, such as the chalk beds in England, and those in 

 Texas, which are very similar to the ooze deposits in the oceans 

 to-day. 



Reef limestone. Another kind of limestone formation is 

 known as reef limestone. This is composed of the mineral re- 

 mains of coral and of shellfish that lived on the coral reefs. 

 The shell limestone, called " coquina," familiar along the 

 Florida coast, is made up of shells of mollusks inhabiting the 

 shallow waters of the ocean. 



