3 o8 GENERAL SCIENCE 



214. Disintegrated Rock. All rock on the earth was 

 at one time igneous like that which flows from active 

 volcanos. This igneous rock was acted upon by the heat 

 of the sun, by water, and by the air until it was broken 

 up so that the flowing water could carry the finer particles 

 and some in a soluble form. These deposits of disinte- 

 grated rock made by the water sometimes became deep 

 enough so that they turned into solid rock again. The 

 rock dissolved in the water served to cement the sand 

 and gravel together the same as when cement is used for 

 making concrete. 



Sand rocks were made mostly of sand cemented 

 together. The kind of sand rock was determined largely 



LIMESTONE MADE OF SHELLS 



by the size of the sand grains and their chemical com- 

 position. Lime rock was formed from the shells of 

 water animals. At one time these animals with shells 

 for protection were more numerous than any other kind, 

 and as they died their shells formed deposits many feet 

 deep in the part of the ocean in which they lived. After 

 many feet of sand and clay were deposited on the top of 

 these shells they were pressed together so solidly that they 

 formed stone, which is our present limestone. Limestone 

 could be made of oyster, clam, and muscle shells because 

 they have the same chemical composition as limestone. 

 Chalk is a form of limestone made of very small shells 

 of animals which lived in the water by millions. If chalk 



