On the Different Floras of the liock-Formations. 321 
long series of ages, during which this successive production 
of the different forms of the vegetable kingdom has taken 
place, into three long periods, which I shall call the reign of 
the Acrogens, the reign of the Gymnosperms, and the reign of 
the Angiosperms. 
These expressions indicate nothing more than the succes- 
sive predominance of each of these three great divisions of 
the vegetable kingdom, and not the complete exclusion of the 
others. Accordingly, in the two former, tlie Acrogens and 
Gymnosperms exist simultaneously, only the former ‘at first 
predominate over the second in number and size, while the 
reverse takes place at a later period. 
But, during these two reigns, the angiospermous vegetables 
appear to me, on’ the contrary, either to be completely want- 
ing, or announced only by a few rare indications of doubtful 
character, and very different from their actual forms, mark- 
ing rather the presence of certain monocotyledons, than that 
of the angiospermous dicotyledons. 
Each of these three kingdoms, thus characterised by the 
predominance of one of the great divisions of the vegetable 
kingdom, is most commonly subdivided into many periods, 
during which very analogous forms, belonging to the same 
families, and often to the same genera, are perpetuated. 
Then these periods themselves comprehend many epochs, 
during which vegetation does not appear to have undergone 
any notable changes; but often materials are still wanting 
to establish these latter subdivisions with precision, either 
because the exact geological position of the beds enclosing 
vegetable impressions is not well determined, or because the 
division of the vegetable species in the different beds of the 
same formation has not been carefully established. Accord- 
ingly, I have no doubt that these different epochs, during 
which vegetation has preserved its characters in an invariable 
manner, will be much more multiplied than in the present 
state of our knowledge, and when materials carefully col- 
lected have accumulated on our hands. 
At present, the following is the general division which I 
think the case admits of— 
VOL. XLVIII. NO. XOVI.—APRIL 1850. x 
