36 AGRICULTURE. 



Many immense limestone beds were accu- 

 mulated from the shells of mollusks and the 

 skeletons, or calcareous plates, of starfishes, sea- 

 urchins, crinoids, and all sorts of lime-secreting 

 animals. The forameniferal oozes formed from 

 the calcareous shells of microscopic, unicellular 

 animals of the deep sea have a vast geograph- 

 ical extent.* 



Siliceous Deposits. The Radiolaria are a 

 group of microscopic animals which make sili- 

 ceous secretions instead of calcareous ones. 



Phosphate Deposits. These are terrestial for- 

 mations derived principally from guano, which 

 is composed of the excrement, bones, and re- 

 mains of birds (or in caves, bats). They are 

 found in rainless regions, like Peru and its 

 islands. When the guano is deposited over 

 limestone it gradually changes the limestone 

 from a carbonate to a phosphate of lime. 



3. Environmental Changes. Beavers build 

 dams across streams, and sometimes flood many 

 acres of lowland. By felling trees they inter- 

 rupt the drainage, thus forming marshes favor- 

 ing the formation of peat beds. 



Man also may change natural conditions, 

 either purposely or incidentally, by planting or 

 destroying trees, thus causing the protection 

 (Fig. 11) or denudation of hillside slopes; by 



Jordan and Kellogg's Animal Life, p. 18. 



