- THE SCENERY OF THE COAL PERIOD. 155 



rme creatures swarmed, and lived, and died upon the 

 grounds that had often aforetime been the seat of terres- 

 trial vegetation. Thus, perhaps, a bed of calcareous sedi- 

 ments, destined to become a limestone, was interpolated 

 among the couches of sand, and shale, and vegetable mat- 

 ter. 



The theatre of these changing scenes was the whole of 

 that area now covered by the coal-measures of the country 

 (see Fig. 58), as well as large portions of the intervening 

 regions, from which the coal has been swept by the besom 

 of geological denudation. In the later ages of geological 

 history, wasting agencies have moved over the surface of 

 the country, scoring through the solid rocks, scooping out 

 lake-basins, carrying away entire formations, and exposing 

 deeply-seated strata over wide areas. 



The duration of the vicissitudes which I have sketched 

 was inconceivably great. The amount of vegetable matter 

 in a single coal-seam six inches thick is greater than the 

 most luxuriant vegetation of the present day would fur- 

 nish in twelve hundred years. Boussingault calculates 

 that luxuriant vegetation at the present day takes from 

 the atmosphere about half a ton of carbon per acre annu- 

 ally, or fifty tons per acre in a century. Fifty tons of 

 stone-coal, spread evenly over an acre of surface, would 

 make a layer of less than one third of an inch. But sup- 

 pose it to be half an inch ; then the time required for the 

 accumulation of a seam of coal three feet thick — the thin- 

 nest which can be worked to advantage — would be seven 

 thousand two hundred years. If the aggregate thickness 

 of all the seams of coal in any basin amounts to sixty feet, 

 the time required for its accumulation would be one hun- 

 dred and forty-four thousand years. In the coal-measures 

 of Nova Scotia are seventy-six seams of coal, of which one 

 is twenty-two feet thick, and another thirty-seven. The 



