STRATA CHARACTERISED 
205 
With the aid of the above lists a direct 
comparison may be made between the ac¬ 
cumulations of fossil plants forming coal in 
the early and middle epochs ; and by ex¬ 
tending our observation to the Rhine we 
find in the tertiary brown coal of that local¬ 
ity, a third term in the series. On examin¬ 
ing these coal fields thus occurring in the 
three great periods in geological time, and 
not far distant from each other in geograph¬ 
ical space, we discover that there is no 
species of plants common to the three for¬ 
mations, or even found in any two of them. 
Each is distinguished from the other by an 
invariable difference in species, as well as by 
the prevalence of distinct tribes of the 
vegetable kingdom. 
Thus where the circumstances have been 
in each case equal to the production of a 
profuse vegetation, and even when we find 
some families of plants, e,g, pines and ferns, 
persistent throughout more than one epoch, 
yet there is a total difference of species 
between the plants found fossil in the coal 
of the three localities. To this remark an 
apparent exception occurs in the plants 
u 5 
