614 DYNAMICAL GEOLOGY. 



ference. Over many parts of New England and other portions of 

 North America there are extensive beds. The amount in Massa- 

 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 Cyclas and 

 Planorbis — which were living in the lake. There are often also 

 beds of the siliceous shields of Diatoms. 



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 of peat, and is used 

 especially when the material is employed as a manure. It includes also impure 

 varieties not fit for burning, being applied to any black swamp-earth consisting 

 largely of decomposed vegetable matter. 



Peat-beds sometimes contain standing trees, and entire skeletons 

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

 often an antiseptic power, and flesh is sometimes changed by the 

 burial into adipocere. 



2. Coral Formations. 



Coral formations are made through the growth mainly of coral 

 zoophytes, 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 G8° 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 map). (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 sub- 

 marine 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 100 feet. 



