uminous coal must have been
formed, not under water, but on dry land.

I have dwelt at length on these further arguments, because they seem
to me as pretty a specimen as I can give my readers of that regular
and gradual induction, that common-sense regulated, by which
geological theories are worked out.

But how does this theory explain the perfect purity of the coal? I
think Sir C. Lyell answers that question fully in p. 383 of his
"Student's Elements of Geology." He tells us that the dense growths
of reeds and herbage which encompass the margins of forest-covered
swamps in the valley and delta of the Mississippi, in passing through
them, are filtered and made to clear themselves entirely before they
reach the areas in which vegetable matter may accumulate for
centuries, forming coal if the climate be favourable; and that in the
cypress-swamps of that region no sediment mingles with the vegetable
matter accumulated from the decay of trees and semi-aquatic plants;
so that when, in a very dry season, the swamp is set on fire, pits
are burnt into the ground many feet deep, or as far as the fire can
go down without reaching water, and scarcely any earthy residuum is
left; just as when the soil of the English fens catches fire, red-hot
holes are eaten down through pure peat till the water-bearing clay
below is reached. But the purity of the water in peaty lagoons is
observable elsewhere than in the delta of the Mississippi. What can
be more transparent than many a pool surrounded by quaking bogs,
fringed, as they are in Ireland, with a ring of white water-lilies,
which you dare not stoop to pick, lest the peat, bending inward,
slide you down into that clear dark gulf some twenty feet in depth,
bottomed and walled with yielding ooze, from which there is no
escape? Most transparent, likewise, is the water of the West Indian
swamps. Though it is of the colour of coffee, or rather of dark
beer, and so impregnated with gases that it produces fever or cholera
when drunk, yet it is--at least when it does not mingle with the salt
water--so clear, that one might see every marking on a boa-
constrictor or alligator, if he glided along the bottom under the
canoe.

But now comes the question--Even if all this be true, how were the
forests covered up in shale and sandstone, one after another?

By gradual sinking of the land, one would suppose.

If we find, as we may find in a hundred coal-pits