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COAL AND ITS CLASSIFICATION 261
ture—and by change into a more compact, heavier, and
more brittle substance.

Humic Series.

——

0

Wood . ,
Peat . . .
Lignite . .
Bituminous coal . .
Steam v '
Anthracite (Pennsylvanian)

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[2°2 83
9:6 557
75 6o
6:6 93
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Sapropelic,

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Wigan Cannel

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10°11

Pear—Peat represents the first stage in one method of coal
formation. It is soft, brown or black, and varies in texture
from a fibrous material in which the plants can be recognized
to structureless jelly. It is an accumulation of vegetation
due to the simultaneous growth and decay of plants when
saturated with water, which prevents their complete decomposition
 into water and carbon dioxide. The plants at the
upper part of the deposit decay under the action of the oxygen
of the air and of bacteria and fungi; but in the lower layers,
as air is excluded and the conditions are aseptic, chemical
decomposition is prevented, and the materials accumulate
by the continued growth of the plants above. The name
probably comes from a Celtic word meaning pieces, as peat
can be pulled into shreds of vegetation; it was known in
England as turf until the name peat spread from Scotland
late in the eighteenth century.
Peat is usually formed on cold moorlands by the growth of
mosses and rushes. It is most abundant between 35° and
60° N. where the mean annual temperature is from 40° to
60° F. Its close dependence on this temperature probably
explains why its formation has ceased on some Scottish moorlands,
 though in Ireland and Germany it grows at the rate of
a foot in from 5 to 10 years. Peat is comparatively rare in
warmer countries, where plant decay is usually complete ;
it occurs in Italy, East Africa, Madagascar, and such places