CARBONIFEROUS PERIOD 51 



and smaller growths in their generations flourished and died ; 

 and from their accumulated remains, deep and extensive 

 peat-bogs were formed. 



In course of time, owing to land oscillations, these peat- 

 bog areas formed mainly in large deltas and estuaries 

 gradually sank below the water-level, and became covered 

 with muddy deposits. 



Coal was therefore in promise ; but the peaty material 

 had to be submitted to tremendous pressure from above, and 

 to undergo various chemical changes, before attaining that 

 condition. This was to be the work of Time. 



As subsidence continued, an ever-increasing pressure 

 resulted from the accumulations of clay, sand, and other 

 earthy material deposited by rivers on the buried peat. In 

 some cases these muddy deposits were of enormous thickness, 

 indicating subsidence of very long duration. 



After the downward movement had ceased, river-borne 

 deposits must have gone on accumulating, until they ap- 

 peared well above the surface of the waters. Whilst in some 

 regions the reappearance of land was doubtless accelerated 

 by earth movements resulting in actual upheavals. 



Vegetation, no doubt, quickly spread over the new land : 

 and in course of time dense brakes and forests held the ground 

 beneath which the earlier sylvan wealth lay buried and 

 compressed. 



Owing to a repetition of land oscillations the peat-bogs, 

 formed on these scenes of renewed verdure, experienced 

 in their turn the same fate as those of earlier formation. 

 The marshy areas sank below the water-level, and were 

 gradually overspread with mud and sand. Thus the material 

 for a second bed of coal, separated from the first by river- 

 borne deposits, and itself buried in like manner, became 

 stored in the strata. 



In some regions these up-and-down movements continued 

 for an inconceivably long time, resulting in the entombment 

 of a series of peat-accumulations. The intervals in the 

 process, no doubt, varied greatly, for the layers of coal and 

 the intervening muddy deposits are of divers degrees of 

 thickness. 



