COAL 2 I 5 



certain fresh-water formations, as in Wyoming. Microscopic 

 examination sometimes reveals the presence of sponge spicules 

 and other siliceous organisms, but this is by no means always the 

 case. As we have seen, the structureless cherts are believed to 

 have been formed by chemical precipitation (p. 212). 



c. Ferruginous Accumulations 



The iron deposits which can be referred to the activity of living 

 creatures are of small extent and importance, but certain of the 

 bog-iron ores are believed to be due to the agency of diatoms, 

 which extract the iron from its dissolved state. 



d. Carbonaceous Accumulations 



The rocks of this group are formed, almost entirely, by the 

 accumulation of vegetable matter and its progressive, though incom- 

 plete, decay under water. This decay is of such a nature that the 

 gaseous constituents diminish, while the carbon is removed much 

 less rapidly, consequently the proportion of the latter substance 

 steadily rises. All the varieties of carbonaceous rocks pass into 

 one another so gradually, that the distinction between them seems 

 somewhat arbitrary. From fresh and unchanged vegetable matter 

 to the hardest anthracite there is an unbroken series of transitions. 



Peat is a partially carbonized mass of vegetable matter, brown 

 or black in colour and showing its vegetable nature on the most 

 superficial examination, though the parts which have been longest 

 macerated are often as homogeneous and as fine grained as clay, 

 and reveal their true nature only under the microscope. 



Lignite or Brown Coal fs a brown or black mass of mineralized 

 and compressed peat, and though still plainly showing its vegetable 

 nature, it does so less obviously than peat, being more carbonized. 

 It is an inferior fuel, though often very valuable in regions where 

 other fuel is scarce or entirely wanting. 



Coal is a compact dark brown or black rock, in which vegetable 

 structure cannot be detected by the unassisted eye, though micro- 

 scopic inspection seldom fails to reveal it. Coal is found in beds or 

 strata, interstratified with shales, sandstones, and, less commonly, 



