48 THE STORY OF Tin: EARTH. 



all give evidence of shallow-water conditions, 

 e\a< tly SUCh as are seen in the growths of fring- 



reefs of coral at the present day. 



There is a second group of limestones which 

 may be termed oceanic, or deep-sea limestones, 

 made known by exploration of the floors of the 



it oceans. They are largely composed of the 

 minute organisms termed foraminifera, such as 

 cover so much of the Atlantic, Pacific and Indian 

 Oceans. Among geological deposits due to such 



misms, may be placed the Chalk of Europe; 



the Nummulitic limestone, which extends through 

 Europe, Africa, and Asia; and the Fusulina lime- 

 stone, which extends from Russia to Japan. 



There are other animals which appear to form 

 deep-water limestones, since they live in some 

 depth of water at the present day, such as the 

 group of shells termed Brachiopoda; and Kncri- 

 nites, which make up portions of the Carbonifer- 

 ous Limestone in this country. 



An oceanic limestone is not necessarily built 

 up in deep water, although such r0< ks often attain 

 a great thickness. It is only necessary that the 



in which the rock accumulates should be be- 

 yond the limits to which sediments from land can 



be transported. Ocean basins often increase in 

 depth as the deposit m< reases m thickness, when 



limestones niav be formed as thick as are the 



Carboniferous limestone of Flint and Derbyshire. 



; ! ,// > Deposits. 

 Accumulations of sands, i lays, and limestones 



brought down from higher land when 

 Cumulate upon the land sun 



If the pond 01 upon a limestone the 



