

CARBONIFEROUS PERIOD. 361 



rushes and many other plants; and this would remain in the 

 coal. Potash is present in all vegetation ; but, as its salts are solu- 

 ble, it would mainly disappear in the course of the decomposition. 

 Traces of sulphur occur in all vegetable matters as well as animal, 

 whether microscopic or not, which might, therefore, be present in 

 the accumulating beds ; and this sulphur, by combination with iron, 

 would have formed pyrites, — a common impurity in coal beds. 



Impurities were also introduced as earth or clay. Even the winds 

 transport dust, and the waters carry detritus. Both of these means 

 may have contributed to the earthy ingredients of the coal. 



Waters may also bring in other ingredients in solution, as oxyd of 

 iron in combination either with carbonic acid, sulphuric acid, or 

 some organic acid ; for iron is carried in these ways (mainly the 

 last) into all marshy or low regions from the hills around, being 

 derived from the decomposition of pyrites (a sulphuret of iron) and 

 other iron minerals. 



Sulphate of iron would lose its oxygen from contact with decom- 

 posing vegetation, and become sulphuret of iron or pyrites ; and 

 this is another source of pyrites. In the change, the oxygen takes 

 carbon from the coal or decomposing plants, and forms carbonic 

 acid, which passes off into the air, and leaves only sulphur and 

 iron, to make sulphuret of iron, or pyrites. 



(6.) Coal-making decomposition takes place only under water. — Where 

 vegetation decomposes in the open air, all the carbon enters into 

 gaseous combinations, and is lost in the atmosphere, only traces 

 remaining to give a dark color to the soil. Hence forests may, 

 with each autumn, drop tons of solid material to the ground, age 

 after age, and yet little remain behind to indicate the existence 

 of that vegetation. But where the bed of leaves and other relics 

 of the plants is covered by water, so that the air is mostly excluded, 

 the decomposition is less complete, — precisely as when wood is 

 charred in a half-smothered fire ; a part of the carbon remains behind, 

 and forms coal. These principles are sustained by facts in all parts of 

 the world. Hence, if a continent were spread equally with vegetation 

 from the equator to the poles, it would form and preserve beds of vege- 

 table debris and fossils only in its marshy regions, or where the relics 

 had been swept off into the waters and had there become buried. 



The peat-beds of the present day much resemble in origin the an- 

 cient plant-beds. (See p. 613.) 



2. Climate, Atmosphere. — The growth of the Carboniferous 

 vegetation was dependent, as now, on the climate and the condition 

 of the atmosphere. 



(1.) Temperature of the ocean and air. — In the animal life of the 

 waters we have a safe criterion for the temperature of the oceans. 



