17 
regards the profusion of the individual species that did exist, 
there can be but one opinion; and in order to form a just 
conception of the luxuriance of vegetation at remote periods 
of the world as compared with the present, we have only to 
contemplate the masses of vegetable matter accumulated, and 
which have been converted into coal, covering many thousands 
of square miles in this country, as well as on the continent of 
Europe, measuring many yards in thickness, and several 
times repeated, as at the Saarbruck Coal Field, where there 
are 120 seams lying one over the other, exclusive of a host 
of smaller seams less than a foot in thickness ; and then take 
into account the fact stated by Humboldt, that in the tem- 
perate zone of the present day, the accumulated growth of 
a forest in a hundred years would not, if converted into 
coal, cover the ground on which it stands much above half an 
inch thick ; which teaches us, I conceive, that to profusion 
would most probably be superadded rapidity of growth and 
brevity of duration, features of no slight importance in the 
primaeval flora; as these agencies would accelerate, to a great 
extent, the accumulation of an enormous bulk of carbon, 
from the short period that would hence be required for the 
successive reproduction of large masses of vegetation over a 
vast extent of country. Lastly, as another stimulus to 
excessive luxuriance of growth, I may add Brongniart's con- 
jecture, " That the atmosphere, at the time of the deposition 
of the Carboniferous strata, was far more charged with 
carbonic acid gas than now ; and that it was this which not 
only enabled gigantic species to develop at a time when 
there was little soil to support them, but also in some 
measure prevented their dead remains from being decom- 
posed by the action of the oxygen of the atmosphere." 
The 323 species of plants described from the coal 
measures of this country, (which is far, however, from 
being the actual number known in collections,) are not all 
VOL. III. B 
