PEA 



[344] 



PEC 



concretions. It scratches glass. 

 Specific gravity from 2 '20 to 2-55. 

 "When breathed upon, it frequently 

 gives out an argillaceous odour. 

 Its constituents are silica 77*0, 

 alumina 13'0, lime and natron 2*6, 

 potash 1*4, oxides of manganese 

 and iron 2-0, water 4-0. 



PEA STONE. A variety of limestone, 

 called also pisolite. 



PEAT, (derived by some from the 

 German word pfutze, a pool, or 

 standing water.) An intermediate 

 substance between simple vegetable 

 matter and lignite, the conversion 

 of peat into lignite being gradual, 

 and brought about by the action of 

 water. This substance, arising 

 sometimes from the subversion of 

 forests covered by the sphagnum 

 palugtre, and other mosses, and 

 sometimes from the growth of 

 various maritime and semi-mari- 

 time plants on the marshes border- 

 ing the coasts, is found among the 

 most modern alluvia, generally 

 covering them; often containing 

 works of human art imbedded, and 

 in many instances still in the act of 

 progressive increase. Peat is com- 

 posed of the remains of many 

 different plants, but generally a 

 great portion is derived from the 

 Sphagnum palustre, and the pro- 

 cess by which these vegetables are 

 thus converted is clearly seen in 

 the sphagnum palustre. As the 

 lower extremity of the plant dies, 

 the upper sends forth fresh roots, 

 thus furnishing a perpetual supply 

 of decomposing vegetable matter. 

 Dr. Maculloch states, " where the 

 living plant is still in contact with 

 the peat, the roots of the rushes, 

 and ligneous vegetables, are found 

 vacillating between life and death, 

 in a spongy half decomposed mass. 

 Lower down, the pulverized car- 

 bonaceous matter is soon mixed 

 with similar fibres, still resisting 

 decomposition. These gradually 

 disappear, and at length a finely- 



powdered substance alone is found, 

 the process being completed by the 

 total destruction of all the organised 

 bodies. The generation of peat, 

 when not completely under water, 

 is confined to moist situations, 

 where the temperature is low, and 

 where vegetables may decompose 

 without putrifying." While in the 

 upper portions of a peat bed we 

 may perceive the fibres of the vege- 

 tables, whence it has originated, in 

 an almost unchanged state ; in the 

 middle portions the texture presents 

 itself gradually obliterated, and the 

 mass is found passing into a com- 

 pact peat ; in the lowest part this 

 change is carried still farther, and 

 substances analogous to jet are met 

 with : in some instances beds of 

 peat alternate with beds of mud or 

 sand deposited in lakes, or of silt 

 and sand formed in the estuaries of 

 rivers ; in these cases they repre- 

 sent an imperfect and unmatured 

 coal formation. 



Sir H. Davy states that one 

 hundred parts of dry peat contain 

 from sixty to ninety-nine parts of 

 matter destructible by fire. One- 

 tenth of the whole of the surface of 

 Ireland is stated to be peat. At 

 the bottom of peat-mosses there is 

 occasionally found a cake or pan of 

 oxide of iron ; whence this is de- 

 rived does not appear to be clearly 

 understood. The preservative pro- 

 perty of peat is very remarkable ; 

 bodies of persons who have perished 

 in peat-bogs have been kept free 

 from putrefaction for many years. 



PECOP'TEBIS. A genus of fossil terres- 

 trial plants found in the coal mea- 

 sures. 



PE'CTEN. (pecten, Lat. a comb.) A 

 genus of marine bivalves, belonging 

 to the family Ostracea. The pecten 

 is a fossil as well as a recent shell, 

 many species being found in our 

 seas. It is a regular, eared, longi- 

 tudinally ribbed, inequivalved bi- 

 valve, with contiguous beaks, 



