THE STORY OF OUR ROCK FOUNDATION 



45 



are usually deposited in the ocean nowadays close inshore, unless 

 the streams bringing in the sand are swift or unless rapid ocean 

 currents carry the sand well out from the shore line. Fine mud 

 may be carried in suspension long distances by the water. It 

 settles only in the quiet parts of the ocean, which are usually 

 the deepest ones, or in isolated bayous free from the turbulence 

 of the waves, from currents, and from undertow. Limestone is 

 made up of the more or less completely ground up shells of 

 animals like clams, oysters, and other similar forms, or the hard 



Fig. 29. — A coral reef exposed in the quarry at Thornton, Illinois, Men are 

 standing at the center of the concentric lines of growth. Photo by Link. 



parts of corals or of other animals whose skeletons are composed 

 largely of lime. By pressure and infiltration this pulverized 

 material is later transformed to the limestone. Now animals like 

 corals and clams will not live where the water is very muddy. 

 Corals especially must have fairly deep and clear water in which 

 to thrive. Wherever an area of limestone is found, especially if 

 it contains coral beds in situ, it means that the body of water 

 at whose bottom it formed must have been fairly deep and 

 sufficiently far removed from the shore line to be beyond the 

 deposits of sand or any large amount of mud. 



