theory
as the above to account for the formation of coal-seams. By this
theory, as is obvious, we are compelled to suppose that the vast
alluvial and marshy flats upon which the coal-plants grew were
liable to constantly-recurring oscillations of level, the successive
land-surfaces represented by the successive coal-beds of any
coal-field being thus successively buried beneath accumulations
of mud or sand. We have no need, however, to suppose that these
oscillations affected large areas at the same time; and geology
teaches us that local elevations and depressions of the land
have been matters of constant occurrence throughout the whole
of past time.

All the varieties of coal (bituminous coal, anthracite; cannel-coal,
&c.) show a more or less distinct "lamination"--that is to say,
they are more or less obviously composed of successive thin layers,
differing slightly in colour and texture. All the varieties of coal,
also, consist chemically of _carbon_, with varying proportions of
certain gaseous constituents and a small amount of incombustible
mineral or "ash." By cutting thin and transparent slices of coal,
we are further enabled, by means of the microscope, to ascertain
precisely not only that the carbon of the coal is derived from
vegetables, but also, in many cases, what kinds of plants, and what
parts of these, enter into the formation of coal. When examined
in this way, all coals are found to consist more or less entirely
of vegetable matter; but there is considerable difference in
different coals as to the exact nature of this. By Professor
Huxley it has been shown that many of the English coals consist
largely of accumulations of rounded discoidal sacs or bags, which
are unquestionably the seed-vessels or "spore-cases" of certain
of the commoner coal-plants (such as the _Lepidodendra_). The
best bituminous coals seem to be most largely composed of these
spore-cases; whilst inferior kinds possess a progressively increasing
amount of the dull carbonaceous substance which is known as "mineral
charcoal," and which is undoubtedly composed of "the stems and
leaves of plants reduced to little more than their carbon." On
the other hand, Principal Dawson finds that the American coals
only occasionally exhibit spore-cases to any extent, but consist
principally of the cells, vessels, and fibres of the bark,
integumentary coverings, and woody portions of the Carboniferous
plants.

The number of plants already known t