HOW THE ANCIENT FORESTS BECAME COAL. 59 



an essentially terrestrial formation, peculiar to the recently emerged 

 land of the period. In Belgium and England they rest upon a marine 

 deposit, which forms their floor, and which reappears in the course 

 of the formation, alternating several times with the strata of land- 

 growth. We learn from this that the sea was retiring from these spots 

 before the extension of the continental area, leaving a broader strip 

 of land after each fitful inundation, and that the carboniferous vegeta- 

 tion was developed on the ground which the marine waters had just 

 abandoned. This phenomenon acquires great force with its frequent 

 recurrence and repetition in various places. 



None of the carboniferous plants except the stigmarias, whose 

 peculiarities we have noticed, appear to have been especially aquatic ; 

 but they could all endure the immediate neighborhood and occasional 

 contact of water without being hurt by it, and could live and grow, 

 even when partially inundated. They grew around the borders and 

 on the slopes of the lagoons with which the shore was studded, the 

 smaller ones thickly matted under the cover of the larger trees, in 

 groups characterized by the predominance of single species, as is 

 shown by the distribution of the fossils. The coal was deposited in the 

 lacustrine beds at the center of these forest-covered depressions ; and 

 the extent of the deposits is measured by the area of the basins that 

 were fitted to receive them. One condition was essential, without 

 which no seam of combustible matter could have been formed. It 

 was, that the water flowing over the ground should bring with it and 

 leave in the bottom of the basin where the carboniferous matter was 

 destined to accumulate, only the remains of plants, to the exclusion of 

 every other form of sediment. This condition may have been more 

 easily realized in the Carboniferous epoch than at any other time, be- 

 cause the flora was more abundant and its extension more favored by 

 the climate. It is conceivable also that, after having been once estab- 

 lished, it might have been liable to interruption at any time ; for a 

 slight oscillation of the ground, a change in the direction of the cur- 

 rents, the washing down of a bluff, or the removal of some impediment, 

 may have been enough to furnish an opportunity for the introduction 

 of sand, mud, or rock-dust, into the deposits. We may also affirm as 

 essential that there should be no real affluent coming down to the 

 place of deposit, or current of running water, for that would bring 

 down mud, and leave in the bed some other sediment than one of coal. 

 The flow of water must have been a gentle trickling over the soil, 

 bathing it without washing it, but strong enough to carry along the 

 vegetable matter which it finally deposited. Whenever the flow be- 

 came more violent, the formation of coal was interrupted to give place 

 to deposits of shale or sandstone, according to the character of the 

 mineral elements brought down, or, if they were in relatively small 

 proportion to the vegetable fragments, of schistose laminae marked 

 with impressions of plants. 



