A FEW WORDS ABOUT COAL. 301 



whether the land sank or the sea rose, there must have 

 been a great oscillation of the earth's crust. But, on 

 the other hand, we see that the great swamps must 

 have retained their horizontal position unaltered for 

 long periods of time. The growth of a forest is not 

 the work of a few years, nor could the accumulations 

 of vegetable matter have been formed quickly. As 

 Lyell says, we have ' evidence of the former existence 

 at more than eighty different levels' overlying 

 levels, be it noticed 'of forests of trees, some of 

 them of vast extent, and which lasted for ages, giving 

 rise to a great accumulation of vegetable matter. 

 Under what condition must the earth's crust have been 

 when such processes were possible ? To this question, 

 as yet, geology has given no satisfactory answer. There 

 are considerations, however, which seem at least sug- 

 gestive of a solution of some of the difficulties here 

 presented. 



It is, in the first place, a remarkable circumstance 

 that, although vegetation was certainly not limited to 

 the carboniferous period, yet it was in that period that 

 all the chief coal-fields were formed. There are ex- 

 ceptions, no doubt, to this rule. In the times which 

 preceded the carboniferous period, some coal-seams 

 were formed, and some well-known coal-fields belong to 

 later geological periods. There are beds of true coal 

 belonging to the tertiary period (the latest of the main 

 geological periods) ; and, passing from the oldest tertiary 

 period to our own time, we find instances of the depo- 

 sition of enormous quantities of lignite and brown coaly 



