CORAL FORMATIONS. 617 



chusetts alone has been estimated to exceed 120,000,000 of cords. 

 Many of the marshes were originally ponds or shallow lakes, and 

 gradually became swamps, as the water, from some cause, diminished 

 in depth. The peat is often underlaid by a bed of whitish shell-marl, 

 consisting of fresh-water shells — mostly species of Sphcerium, Lim- 

 ncea, Physa, and Planorbis — which were living in the lake. There 

 are often also, especially in regions of siliceous or metamorphic rocks, 

 beds of a white chalky character, made of the siliceous shields of Di- 

 atoms. 



Peat is used for fuel and also as a fertilizer. When prepared for burning, it is cut 

 into large blocks, and dried in the sun. It is sometimes pressed, in order to serve as 

 fuel for steam-engines. Much is another name for peat, especially impure kinds, when 

 employed as a manure ; any black swamp-earth consisting largely of decomposed 

 vegetable matter is so called. 



Peat-beds sometimes contain standing trees, and entire skeletons of 

 animals that had sunk in the swamp. The peat-waters have an anti- 

 septic power. They consequently tend to prevent complete decay of 

 the vegetable matter of the peat bed ; and flesh is sometimes changed 

 by the burial into adipocere. 



2. Coral Formations. 



Coral formations are made mainly from the calcareous secretions of 

 coral-making polyps, and are confined to the warmer latitudes of the 

 globe. 



Kinds. — Coral formations, while of one general mode of origin, 

 are of two kinds : — 



1. Coral islands. — Isolated coral formations in the open sea. 



2. Coral reefs. — Banks of coral, bordering other lands or islands. 

 Distribution. — The limiting temperature of reef-forming corals is 



about 68° F. ; that is, they do not flourish where the mean tempera- 

 ture of any month of the year is below 68°. The extent of the Coral 

 seas is shown by the position of the north and south lines of 68° F., 

 on the Physiographic Chart, as already pointed out. 



The exclusion of corals from certain tropical coasts is owing to dif- 

 ferent causes. — (1.) The cold extratropical oceanic currents, as in the 

 case of western South America (see chart). (2.) Muddy or alluvial 

 shores, or the emptying of large rivers ; for coral-polyps require clear 

 sea-water and generally a solid foundation to build upon. (3.) The 

 presence of volcanic action, which, through occasional submarine action, 

 destroys the life of a coast. (4.) The depth of water on precipitous 

 shores ; for the reef-corals do not grow where the depth exceeds one 

 hundred feet. 



For the last-mentioned reason, reefs are prevented from commencing 



