THE EARTH'S ROCK FOUNDATIONS 



shells of animals or corals can exist in such quantity and be so 

 ground up as to form great beds of rock, yet the process can 

 readily be seen now going on in many localities, as along the coast 

 of Florida. The bed rock of that state is largely such limestone 

 cochina limestone, of very recent formation and the little clam, 

 the cochina, exists in countless 

 hordes in the ocean along its 

 shores. The area of the state 

 is constantly being thus ex- 

 tended. The soil of the states 

 of the Middle West, Ohio, In- 

 diana, Illinois, etc., lies in large 

 part on a limestone bed rock 

 deposited in the old seas that 

 once covered their present 

 sites. Such beds of limestone, 

 often hundreds of feet thick, 

 represent the accumulated re- 

 mains of untold numbers of 

 shells and countless genera- 

 tions of corals (Fig. 30). But 

 the time consumed in their 

 formation according to the 

 geologists mounts up into the 

 millions of years, which is 

 quite necessary for such a vast 

 procession of living things. 



Limestone may be almost 

 as hard as feldspar or very 

 soft. It can always be scratched with a knife. It may be of 

 many colors though usually it is some shade of yellow or gray. 

 Since it is composed of calcium carbonate it effervesces with acid. 

 It often contains fossils, the remains of animals and plants that 

 were buried in the mud when the limestone was forming and were 

 altered with it to stone. Such fossils show with remarkable 



FIG. 30. Limestone, showing stratification 



