ORGANIC AGENCIES. 91 



4. Even in the most intensely red-clay regions, in 

 wooded places the surface-soil, for a foot or more in 

 depth, is bleached. Water containing organic matter 

 from the surface leaches out and carries down the color- 

 ing iron to the subsoil. 



5. The clay of uplands may be yellow or red, but the 

 clay of swamp-lands is always bluish. This is because 

 ferric oxide, which is the red or yellow coloring-matter, 

 can not exist in the presence of organic matter, abundant 

 in swamps, but is reduced to ferrous carbonate and its 

 color destroyed. But, if such blue clay be burned to 

 brick the organic matter is destroyed, the iron is peroxi- 

 dized, and the brick is red. 



SECTION III. LIME ACCUMULATIONS. 



Lime accumulations are made mainly by corals and by 

 shells. We shall take up the subject under these two 

 heads. 



Coral Reefs and Islands. 



Although corals do not make reefs in temperate 

 regions, and therefore the process can not be observed by 

 every one, yet, for many reasons, the subject is of peculiar 

 interest, both popular and scientific. Coral reefs are of 

 peculiar popular interest 1. On account of the strange 

 forms and gorgeous beauty of the animals which inhabit 

 them. 2. On account of the gem-like beauty of the isl- 

 ands which form on them. 3. Because a large area is 

 added to the habitable land-surface by the agency of 

 corals ; and especially, 4. Because the largest continuous 

 body of land thus added is on our own coast, viz., in 

 Florida. 5. Because of the great dangers to navigation, 

 especially on the coast of Florida, resulting from the 

 presence of these reefs. The considerable town of Key 

 West owes its existence wholly to the wrecking business. 



west 



