THE CARBONIFEROUS PERIOD. 71 
nessivo stages of a history which w T e can read 
only in its results. Shut out from the ocean, 
these shallow sea-basins were gradually changed 
by the rains to fresli-water lakes ; the lakes, in 
their turn, underwent a transformation, becom- 
ing filled, in the course of centuries, with the 
materials worn away from their shores, with the 
debris of the animals which lived and died in 
their waters, as well as with the decaying matter 
from aquatic plants, till at last they were changed 
to spreading marshes, and on these marshes arose 
the gigantic fern-vegetation of which the first for- 
ests chiefly consisted. Such are the separate 
chapters in the history of the coal-basins of Illi- 
nois, Missouri, Pennsylvania, New England, and 
Nova Scotia. First inland seas, then fresh-water 
lakes, then spreading marshes, then gigantic for- 
ests, and lastly vast storehouses of coal for the 
human race. 
Although coal-beds are by no means peculiar 
to the Carboniferous period, since such deposits 
must be formed wherever the decay of vegetation 
is going on extensively, yet it would seem that 
coal-making was the great work in that age of 
the world’s physical history. The atmospheric 
conditions, so far as we can understand them, 
were then especially favorable to this result. 
Though the existence of such an extensive ter- 
restrial vegetation shows conclusively that an at 
