'911] STEVENSON— FORMATION OF COAL BEDS. 575 



frequently falls to powder, v. Giimbel found also, in many peats, 

 deep black coaly parts of plants as small fibers, a form which he 

 terms Torfifaserkohle and regards as a thoroughly characteristic type. 



Peat always contains much water, often 95 per cent., when freshly 

 removed ; but a great part of this evaporates on exposure, there 

 remaining in the air-dried material from 10 to 30 per cent., the denser 

 peat retaining the larger quantity. When first taken out, it is plastic 

 but after thorough drying the plasticity is lost. Peat is very porous ; 

 V. Giimbel subjected Sphagnum peat to a vertical pressure of 6,000 

 atmospheres and reduced 100 centimeters to 17.7. The compressed 

 material was apparently homogeneous, the streak was lustrous and 

 lamination was distinct on the fractured surface. The reduction 

 was due wholly to compression, obliteration of the pores, for, when 

 moistened with water, the mass swelled to practically the original 

 bulk. This condition, however, may not be constant. The writer 

 has some briquetted peat, made under great pressure and moderate 

 temperature, which has no tendency to swell when moistened. It 

 has lost all plasticity and in sixteen years it has shown no change on 

 the brilliant surface at each end. 



Peat, then, consists, aside from introduced sand, clay or calca- 

 reous materials, of more or less changed plant tissues, whose organic 

 texture is still recognizable, and of an enclosing substance derived 

 from complete decomposition of plant tissues, which is originally 

 soluble in water but which, on drying or perhaps on oxidation, 

 becomes insoluble. 



Fuel peat has from i to 25 per cent, of ash. The purest peats 

 contain less mineral matter than is found in the plants whence they 

 are derived ; while on the other hand a peat deposit may pass from 

 pure peat into'carbonaceous mud and thence into muds almost wholly 

 W'ithout trace of carbon. Mills and Rowan'^ have given ultimate 

 analyses of surface and dense peats from two localities in Ireland, 

 which represent the extremes of high grade fuel. 

 In each case, the ash is excluded in calculating the other constituents. 

 The same authors give tw^enty-seven analyses of the ash found in 



" E. J. Mills and F. J. Rowan, " Chemical Technology," Philadelphia, 

 1889, pp. 15-20. 



173 



