ormation of the coal seams. Flat, swampy
coastal plains are believed to furnish the best conditions for thick
accumulation of peat. There is good evidence that most of the deposits
accumulate essentially in place, without appreciable transportation.

In time these surface accumulations of vegetable material may subside
and be buried under clay, sand, or other rock materials. The processes
of condensation begun in the peat bog are then carried further. They
result in the second stage of coal formation, that of _lignite_ or
_brown coal_. This is brown, woody in texture, and has a brown streak.
It has a higher percentage of fixed carbon, and less volatile matter and
water, than peat.

Continuation of the processes of induration produces _subbituminous
coal_, or _black lignite_, which is usually black and sometimes has a
fairly bright luster. It is sometimes distinguished from bituminous
coal, where weathered or dried, by the manner in which it checks
irregularly or splits parallel to the bedding,--the characteristic
feature of bituminous coal being columnar fracture.

The next stage in coal formation is _bituminous coal_. It has greater
density than the lignites or subbituminous coals, is black, more
brittle, and breaks with a cubical or conchoidal fracture. It is higher
in fixed carbon, lower in volatile matter and water. A variety of
bituminous coal, called _cannel coal_, is characterized by an unusually
high percentage of volatile matter, which causes it to ignite easily.
This material has a dull luster and a conchoidal fracture. It is
composed almost entirely of the spores and spore cases, which are
resinous or waxy products, of such plants as lived in the parent coal
swamp.

There are gradations from bituminous coal into _anthracite coal_.
_Semibituminous_ and _semianthracite_ are names used to some extent for
these intermediate varieties. The final stage of coal formation is
anthracite,--hard, brittle, black, with high luster and conchoidal
fracture. It has a higher percentage of fixed carbon and correspondingly
less of the volatile constituents, than any of the other coals.

The coals form a completely graded series from peat to the hard
anthracite. Comparison of the compositions of the coal materials at
different stages shows clearly what has happened. Moisture has
diminished, certain volatile hydrocarbons have been eliminated as gases,
and oxygen has decreased. On the other hand, the residual fixed carbon,
s