V ON THE FORMATION OF COAL 157 



time which the coal-field represents would be 

 25,000 x 240 = 6,000,000 years. As affording a 

 definite chronology, of course such calculations as 

 these are of no value ; but they have much use in 

 fixing one's attention upon a possible minimum. 

 A man may be puzzled if he is asked how long 

 Rome took a-building ; but he is proverbially safe 

 if he affirms it not to have been built in a day ; 

 and our geological calculations are all, at present, 

 pretty much on that footing. 



A second consideration which the study of the 

 coal brings prominently before the mind of any one 

 who is familiar with palaeontology is, that the 

 coal Flora, viewed in relation to the enormous 

 period of time which it lasted, and to the still 

 vaster period which has elapsed since it flourished, 

 underwent little change while it endured, and in 

 its peculiar characters, differs strangely little from 

 that which at present exist. 



The same species of plants are to be met with 

 throughout the whole thickness of a coal-field, and 

 the youngest are not sensibly different from the 

 oldest. But more than this. Notwithstanding 

 that the carboniferous period is separated from us 

 by more than the whole time represented by the 

 secondary and tertiary formations, the great types 

 of vegetation were as distinct then as now. The 

 structure of the modern club-moss furnishes a 

 complete explanation of the fossil remains of the 

 Lepidodendra, and the fronds of some of the ancient 



