122 



CARBON AND ITS COMPOUNDS 



by boiling it. This drives off the carbon dioxide, and the 



calcium carbonate then settles out. 



Underground water charged with carbon dioxide wears away 



limestone, forming caves. Sometimes these are very large. When, 

 later, water containing carbon dioxide and 

 limestone enters a cave, the carbon dioxide 

 usually escapes, and the dissolved limestone 

 is deposited. If the water drips from the 

 ceiling of the cave, each drop leaves its bit of 

 limestone behind. In this way hanging masses, 

 called stalactites, are formed. The limestone 

 deposited on the floor forms a rising column, 

 called a stalagmite. Often the two join to 

 form a pillar (c/. Fig. 105). 



Marble is a finely crystallized form 

 of limestone. Limestone makes up not 

 only large rock deposits, but also the 

 shells and hard parts (bones, etc.) of 



animals (Fig. 106). Enormous deposits of limestone (Fig. 



107) are formed out of coral, the stony framework of the 



coral animal (c/. Fig. 257, 334). 



Fia. 106. 



Chalk, as magnified by a 

 microscope. Note the 

 tiny shells it contains. 

 Chalk is not ordinary 

 blackboard crayon. 



FIG. 107. 

 Eagle Cliff, Peninsula Park, Wisconsin. Note the layer formation of the limestone. 



