

AGE OF COAL. 219 



see, of a most luxuriant and profuse character. Trees 

 and shrubs dropped their leaves and fruit year after year, 

 and at length died themselves, while other trees and 

 shrubs took their places ; and thus growth and decay 

 went on, perhaps, through many slowly-moving centu- 

 ries, till enough of these vegetable remains was accumu- 

 lated to make a coal-bed. At length a subsidence of the 

 land took place, the water flowed over it, covering up all 

 this accumulation of vegetable substance, and under the 

 water and the detritus which the water brought in, the 

 decomposition occurred which resulted in the produc- 

 tion of coal. The detritus went on accumulating, at the 

 same time slowly solidifying from below upward, till at 

 length the subsidence ceased, and there succeeded an el- 

 evation, so as to bring the surface again into the swampy 

 condition for a new growth, preparatory to another bed 

 of coal. The set of processes thus described was in 

 some cases many times repeated, there being often many 

 beds of coal between the strata of rock. In Kentucky 

 there are from fifteen to twenty separate coal-beds in 

 the strata, and in Nova Scotia, at a locality called the 

 Joggins, there are seventy-six coal-seams, some of them 

 being very thin. The process was essentially the same 

 for each of these, the periods of elevation and subsidence 

 being longer for the thick seams than for the thin ones. 

 Though there were alternate subsidence and elevation, 

 " the sinking condition," remarks Hugh Miller, " was the 

 general one ; platform after platform disappeared, as 

 century after century rolled away, impressing upon them 

 their character as they passed ; and so the coal-meas- 

 ures, where deepest and most extensive, consist, from 

 bottom to top, of these buried platforms, ranged like the 

 sheets of a work in the course of printing, that, after be- 

 ing stamped by the pressman, are then placed horizon- 

 tally over one another in a pile." 



317. Impurities of Coal. Coal varies much in its pu- 

 rity, as every one who has burned coal must have no- 



