Geography of Ohio 179 



any further tilting that would tend to deepen the water, the sedi- 

 ments in time make it sufficiently shallow for the marsh plants to 

 again spread. The period of deep water is indicated by the clastic 

 bed of sand or conglomerate, overlying the coal bed. The re- 

 currence of swamp conditions is indicated by another coal seam. 

 Again, the manner in which coal seams converge, when worked 

 laterally, is another evidence of the alternation of swamp and 

 sedimentation conditions . 



During the Pennsylvanian period, it is supposed that the land 

 areas in general were low, and that they were bordered by wide 

 tracts of shallow water. The conditions of climate appear to 

 have favored the growth of plants. That plant growth was not 

 exactly analogous to the tropical vegetation of to-day, is inferred 

 from a study of the coal itself and the fossils preserved in it. Out- 

 side of the Mangro\'e swamps of Florida, we do not, at present, 

 have any large trees adapted to swamp habitats. The mosses and 

 other of the lower plant forms to-day thrive best in swamp habitats. 



The important factoi-, howe\'er, in the development of coal 

 seams from vegetation, is the arrested decay that follows the 

 assembling of plant remains in water or moist horizons. Beneath 

 the water putrefaction or fermentation proceeds slowly. The 

 oxygen is gradually given off, but the carbon is retained. The 

 details of the change from wood to the different forms of coal are 

 best understood by consulting the following table:-* 



CARBOX HYDROGEX OXYGEN NITROGEN 



1. Wood 49.66 6.21 13.03 1.1 



2. Peat 59.5 5.5 33,0 2.0 



3. Brown coal 68.7 5.5 25.0 0.8 



4. Bituminous coal 81.2 5 5 12.5 0.8 



5. Anthracite 95.0 2.5 2.5 0.0 



The increasing percentage of carbon, it must be remembered, is 

 relative. The actual amount of carbon does not gain as plants 

 become peat, brown coal, bituminous, etc. The decrease in the 

 amount of oxj^gen, through slow decay, correspondingl}' increases 

 the percentage of the carbon. 



Kinds of coal. The above table shows that coals are classified 

 in accordance with their content of carbon. Sometimes coal is 

 said to be metamorphosed wood. The conditions that bring about 



2* Clianil)erlin and Salisl)ury. Crohxjii. vol. ii, (1906), p. 570. 



