THE CRUST OF THE EARTH 181 



brown color. Other sandstones are light colored, the cement 

 being silica or bicarbonate of lime. 



Shales are consolidated clay beds or mud beds. They 

 often contain mica flakes. As their chief mineral is feldspar, 

 shales decompose readily and make good soil. 



FIG. 91. LIMESTONE QUARRY NEAR. COLUMBUS, OHIO 



1. Tell what you can learn by observation about the formation of 

 this rock and any changes that have occurred in it since it was formed. 

 2. What shows this to be a disused quarry? 



Limestone rock is made from animal skeletons, such as 

 shells and coral. The animals get carbonate of lime of 

 which the skeleton is composed from the waters of the 

 ocean where it is dissolved. When the animals die, their 

 skeletons are ground up by the action of the waves. The 

 white sand and mud so formed are very fine-grained, and 

 when consolidated make limestone. Sometimes the lime- 



