322 STKATIGEAPHICAL GEOLOGY 



plants which, like modern mangroves, did not mind having their 

 roots covered by salt water. 



So far as to general conditions, but we have still to deal with 

 the formation of the individual coal-seams, and to consider the way 

 in which the coal-material may have been accumulated so as to 

 form a seam or bed, for these are sometimes of great thickness, as 

 much as 20 or 30 feet. There are a certain number of facts 

 which seem to indicate that the vegetation from the decay of 

 which a bed of coal was formed grew on the spot, while other 

 facts have suggested the theory of drift-origin, i.e. that the vegetable 

 materials were drifted by a current into their present situation. 



Some writers have maintained the one, and some the other 

 theory ; but as Mr. Arber remarks, it is very likely that both 

 theories are equally true, some coals having been formed in the 

 one way and some in the other. He also points out that the 

 significance of certain facts, often appealed to as evidence for one or 

 the other theory, has been greatly exaggerated and misunderstood. 



Thus the presence of an underclay containing many Stigmarian 

 rootlets is no proof that the coal above has been formed in situ 

 from plants which grew in that clay ; neither does the fact that 

 the roof-bed of a coal contains drifted plant remains prove the 

 coal to have been formed by an accumulation of such drift. The 

 formation of each bed of coal must be individually considered, and 

 its own structure must especially be ascertained and noted ; coals 

 which consist entirely of one kind of material, such as the Cannels, 

 were doubtless formed in situ from local growths, while some of 

 those which consist of many sorts of plants may have been formed 

 of drifted materials. Some seams may have been semi-terrestrial 

 growths like modern peats, others were formed on the floors of 

 large swamp lakes or in brackish-water lagoons, and in some cases 

 these water spaces were clearly near to the estuary of a large river. 



Finally, a few remarks may be made on the succession of plant- 

 assemblages in the Coal-measures. That there is such a succession 

 in the northern part of England admits of 110 doubt, and this same 

 succession prevails over a large area, but whether it prevailed all 

 over Europe, wherever the sequence was complete, has not yet been 

 proved. Thus we have seen that what is known at present as the 

 flora of the Lower Coal-measures cannot be recognised in South 

 Wales, nor in the Bristol and Somerset coalfields, and further that 

 it has not been found on the Continent except in a narrow zone of 

 so-called Millstone grit. From the absence of this particular 

 assemblage of plants, or rather from the absence of certain species, 

 it has been inferred that the Lower Coal-measures are not 

 represented in the more Southern areas. 



