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_).
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