. By the progressive elimination of oxygen
and hydrogen, partly as water and partly as carbon dioxide and marsh
gas, the ratios of carbon to oxygen and hydrogen in the rendered product
increase in the following manner:--

C : H C : O
Cellulose 7.2 0.9
Peat 9.8 1.8
Lignite, imperfect 12.2 2.4
" perfect 12.6 3.6

The resulting product is a brown pasty or gelatinous substance which
binds the more resisting parts of the plants into a compact mass. The
same observer considers Boghead coal, kerosene shale and similar
substances used for the production of mineral oils to be mainly
alteration products of gelatinous fresh water algae, which by a nearly
complete elimination of oxygen have been changed to substances
approximating in composition to C2H3 and C3H5, where C : H = 7.98 and C
: O + N = 46.3. In cannel coals the prevailing constituents are the
spores of cryptogamic plants, algae being rare or in many cases absent.
By making very thin sections and employing high magnification (1000-1200
diameters), Renault has been enabled to detect numerous forms of bacilli
in the woody parts preserved in coal, one of which, _Micrococcus carbo_,
bears a strong resemblance to the living _Cladothrix_ found in trees
buried in peat bogs. Clearer evidence of their occurrence has, however,
been found in fragments of wood fossilized by silica or carbonate of
lime which are sometimes met with in coal seams.

The subsequent change of peaty substance into coal is probably due to
geological causes, i.e. chemical and physical processes similar to those
that have converted ordinary sediments into rock masses. Such changes
seem, however, to have been very rapidly accomplished, as pebbles of
completely formed coal are commonly found in the sandstones and coarser
sedimentary strata alternating with the coal seams in many coalfields.

The variation in the composition of coal seams in different parts of the
same basin is a difficult matter to explain. It has been variously
attributed to metamorphism, consequent upon igneous intrusion, earth
movements and other kinds of geothermic action, greater or less loss of
volatile constituents during the period of coaly transformation,
conditioned by differences of permeability in the enclosing rocks, which
is greater for sandstones than for argillaceous strata, and other
causes; but none of these appears t