ulphur, and usually ash, have remained in higher percentage. This
change in composition is graphically represented in Figure 6.

During this process volume has been progressively reduced and density
increased. Five feet of wood or plant may produce about one foot of
bituminous coal, or six-tenths of a foot of anthracite.

The exact physical conditions in the earth which determine the
progressive changes in coals, above outlined, cannot be fully specified.
Time is one of the factors--the longer the time, the greater the
opportunity for accomplishing these results. Another factor is
undoubtedly pressure, due to the weight of overlying sediments, or to
earth movements. In peat condensational changes of this nature are
accomplished artificially by the pressure of briquetting machines.
Another factor is believed to be the heat developed by earth movements
and vulcanism, which presumably facilitates the elimination of volatile
materials, and thus accelerates the gradational changes above
described. This is suggested by the fact that in places where hot
volcanic lavas have gone through coal beds they have locally produced
coals of anthracitic and coke-like varieties. In general, however, it
has not been possible to determine the degree to which heat has been
responsible for the changes. Coals which have been developed in
different localities, under what seem to be much the same heat
conditions, may show quite different degrees of progress toward the
anthracite stage. Another factor that has been suggested as possibly
contributing to the change, is the degree of permeability of the rocks
overlying the coal to the volatile materials which escape from the coal
during its refinement. It is argued that in areas of folding or of
brittle rock where the cover is cracked, volatile gases have a better
chance to escape, and that the change toward anthracite is likely to
advance further here than elsewhere.

Bacterial action is an important factor in the earlier stages, in the
partial decay of vegetable matter to form peat; accumulation of waste
products from this action, however, appears to inhibit further bacterial
activity.

Coal deposits have the primary shapes of sedimentary beds. They are
ordinarily thin and tabular, and broadly lenticular,--on true scale
being like sheets of thin paper. At a maximum they seldom run over 100
feet in thickness, and they average less than 10 feet. Seldom is a
workable coal bed entirely alone; ther