SEDIMENTARY AND METAMORPHIC ROCKS 93 
substances from the waters which they draw out of the 
soil through their roots. These substances they build 
into their structure ; but when the plants die, they are 
in large part returned to the air or earth. In favor- 
able places, particularly in swamps, where decay is 
retarded, the plant remains may accumulate in beds 
Fic. 39. 
Piece of dried peat, showing cast of roots. Made entirely of plant fragments. 
of peat (Fig. 59), and these may later become trans- 
formed to coal or mineral fuel, which is mainly made 
of carbon. 
Between the peat, which we see forming in the 
swamps, and the hardest anthracite coal, there is every 
gradation. Liagnite or brown coal is a form of fuel 
