104 THE PLANT CELL WALL 



coal which contains less water and ash, and may be somewhat richer 

 in carbon than the lower ranks. They are progressively harder 

 and lower in their content of water and other volatile substances. 

 Peat may contain as much as 74 per cent non-aqueous volatile 

 components whereas bituminous coal contains no more than 

 40 per cent. Semi-bituminous coals which contain no more than 

 20 per cent volatile matter grade into semi-anthracites (approxi- 

 mately 10 per cent volatiles or less). 



The final stage in coal evolution, anthracite is distinguished 

 by its hardness, low volatile content and extremely high carbon 

 content. 



Coalification 



Although it has been suggested that coal might originate from 

 sedimented vegetable matter removed from its point of origin 

 {drift or allochthonous theory), the weight of evidence supports 

 the in situ or autochthonous theory. Accordingly, coal is believed 

 to have originated from the vegetable sediments which character- 

 ize peat bogs and freshwater swamps. Deposits which develop in 

 poorly drained cool moist regions originate in ponds a few hundred 

 feet to a mile in diameter. 



In such areas, succession leads to the establishment of pond 

 weeds, water lilies, rushes, algae and the formation of a floating 

 mat. The accumulation of vegetable debris from these mats slowly 

 fills in the center of the pond while the death of fringe growths 

 leads to organic deposits at the bog edge. In time, peat and muck 

 fill in the bog and allow the formation of conifer stands (tamarack, 

 spruce) and, subsequently decidous trees such as white birch. 



Although typical peat bogs are an important site for formation 

 of lower rank coal progenitors, the freshwater swamps are the 

 most likely source of large coal deposits. Areas such as the Dismal 

 Swamp of North Carolina and the great Sumatra Swamp of the East 

 Indies are probably representative in part of typical sites of coali- 

 fication. In the Dismal Swamp, a peat deposit ranging from 1-20 ft 

 in thickness (average, 7 ft) is contained in an area of 1500 square 

 miles. It is estimated that nearly 700 million tons of peat are held 

 in this deposit, corresponding to a coal seam 1-20 in. in thickness. 



