DECOMPOSITION OF ORGANIC MATTER. 7 



of air converts organic matter into gaseous products. Heat, in 

 the absence of air, will drive off volatile hydrocarbons, and 

 leave fixed carbon behind ; but this is not coal.- (2) Mouldering. 

 When decomposition takes place with an insufficient supply of 

 oxygen to completely oxidize the organic matter, a residue, 

 rich in carbon, is left, such as vegetable mould. (3) Peatifi- 

 cation. This is carried on under water. At first, the process 



Fig. 10. Thinnfeldia 

 odontopte^roides . 



Fig. 11. Podozamites 

 lanceolatus. 



is similar to mouldering, only with less air available. In the 

 second stage the oxygen is completely cut off. (4). Putrefac- 

 tion. This is decomposition under conditions of material tem- 

 perature when air is absent. (5). Dry rot. 



Coal is undoubtedly due to a series of transformations of 

 vegetable matter. In some cases, its structure can be seen under 

 the microscope, or even with the naked eye, and the transfor- 

 mation, commencing with peat, can be followed in its various 



