6 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



delta of the Mississippi are surrounded by dense growths of 

 reeds, which filter the water from all sand and mud. The strata 

 associated with coal consist chiefly of sandstones and shales. 

 The sandstones frequently show ripple marks, and current bed- 

 ding; the shales are generally laminated,, indicating a slow and 

 tranquil deposition. Investigation strengthens the probability 

 that coal was generally deposited in fresh, and sometimes 

 estuarine waters. There are three successive phases that may 

 be distinguished in the formation of coal first, the accumula- 

 tion of vegetable matter; second, the work done by chemical 

 action under water, including dehydration and deoxidation of 

 the cellulose, during which the mass shrinks from 10 to 30 per 

 cent, of the original volume of accumulated matter; and, third, 

 subsequent reactions underground. The following table gives 



Fig. 7. Eurydesma cordata. Fig. 9. Platyschisma 



oculum. 



a fair idea of the relative proportions of carbon, hydrogen, and 

 oxygen in certain fuels : 



Fuel. Carbon. Hydrogen. Oxygen. 



Wood fibre (cellulose) .. 52.65 5.25 42.10 



Peat 60.44 5.96 33.60 



Lignite 66.96 5.27 27.77 



Brown coal 74.20 5.89 19.91 



Bituminous coal 76.18 5.64 18.08 



Semi-anthracite 90.50 5.05 4.40 



Anthracite 92.85 3.96 3.19 



The steady increase in the amount of residual carbon, and 

 the decrease in the amount of oxygen is plainly seen. Neither 

 vegetable matter at the one end nor anthracite at the other end 

 of the changes cake; while of bituminous coals, having a simi- 

 lar ultimate composition, one may cake and the other not. 



Vegetable matter is capable of decomposing in five differ- 

 ent ways, depending chiefly on the absence or presence of air 

 and water: (1) Destructive distillation. Heat in the presence 



