ORGANIC ROCKS 191 



136. ORGANIC ROCKS 



Wherever the word organic is used, it is to be understood 

 that the material which " organic " describes once possessed 

 life, either vegetable or animal. Many animals and shellfish 

 get their supply for shells from the carbonate of lime which is 

 dissolved in the water. When these animals die, their shells 

 become compacted by pressure of one layer upon the other, 

 and form limestone. Another factor which enters into the 

 formation of rock is the diatoms, a very small plant which 

 lives at the surface of the sea in warm climates, and has a 

 shell-like covering containing silica. Large masses of the dead 

 bodies of these settle to the bottom, likewise forming rock, 

 which is called diatomaceous. 



Coral islands are produced by the gradual accumulation of 

 coral growth. Coral is an animal, growing in a shell, and the 

 remains which we see are merely calcium carbonate with a few 

 other compounds in very small amounts. An acid applied 

 to coral, as well as to all of the organic rocks, except those con- 

 taining silica, sets free carbon dioxide. 



Diatomaceous rock is used for polishing purposes, since the 

 little particles, although very fine, are sharp, and cut away the 

 dirt or tarnished material. It is also used as a sound proofing 

 for deadening sound in buildings, and as a packing for fire 

 proofing and for cold-storage plants. Since it is a very poor 

 conductor of heat, it is valuable for these latter purposes. 



References : 



1. 1304:410-411. Organic Rocks. 



2. 1503 : 202. Coral Reefs. 



3. 1703:200-201. Natural Carbonates. 

 a. 1201 : 85-86. Diatomaceous Earth. 



