11 
That the species of plants found in the shales and sand- 
stones of our coal fields are so few, as compared with those 
now existing, is not remarkable, but rather what we might, 
d priori, expect, if we reason from the aspect which dense 
primaeval forests at the present day exhibit to our view, 
and from whence we are entitled to borrow a retrospect of 
the past. For as Playfair observes, " Amid all the revolu- 
tions of the globe the economy of nature has been uniform, 
and her laws are the only things that have resisted the 
general movement. The rivers and the rocks, the seas and 
the continents, have changed in all their parts ; but the laws 
which direct those changes, and the rules to which they are 
subject, have remained the same." The idea also which 
some persons entertain, that, at the period of the deposition 
of the coal fields, the species of plants existing on the earth 
were few, and consisted of only such as we now find em- 
bedded, no grasses, no flowering plants, and scarcely any 
dicotyledonus plants, is not borne out by the revelations of 
geology. That the cereali and various other plants formed 
the early clothing of the earth is almost certain, though no 
traces of them exist ; but the cause of their disappearance 
is sufficiently obvious, and to which I shall call your attention. 
In the first place, however, I would observe, that in the 
trackless forests of America the aborigines are generally 
white or red pine, black, white, or hemlock spruce, maple, 
beech, birch, and other trees capable of attaining great size, 
which spring up from a bed of black vegetable mould, and, 
from their accumulation, choke up and finally destroy other 
smaller trees. Hence we see that in old forests, trees of 
the largest growth and longest life have a tendency to 
prevail to the exclusion of underwood, and the consequent 
greater increase of mosses, ferns, and lycopodiums, whose 
growth is accelerated by such humid localities, the graves 
of departed giants of the vegetable world, whose trunks 
have mouldered away through successive generations. That 
