270 ECONOMIC GEOLOGY
are of poor quality, such as the Virtuewell Seam in Lanark-
shire; while the adjacent high quality coals show no evi-
dence of growth in place. Coal seams moreover may pass
laterally into shale or sandstone, or may subdivide above
and below a layer of sand or clay-like beds deposited by
water. The thickness of many coal seams is inconceivable
for forest growths. On the estimate that 20 feet of vege-
table residue are required to form 1 foot of coal, the 30-foot
seam at Dudley would have required 600 feet of forest debris ;
some seams in India are 100 feet in thickness, and would
have required a thickness of 2000 feet. The Fushun seam
in Manchuria is more than 200 feet thick. Such thick de-
posits present no difficulty as accumulations of vegetation
carried by streams from forest-clad hills into a deep lake.
The theories of the formation of coal in situ and by drift
both appear true for different fields. In Yorkshire, and in
Silesia, where twenty-seven seams are superimposed and each
has its underclay, and in the South Joggins section in Nova
Scotia, where repeated seams with vertical tree-trunks occur
over clay with roots, the coal has been clearly formed as a
forest growth ; but in some fields, as in Scotland, India, and
France, some seams were formed by accumulations of drifted
vegetation.
CarBon EnricuMeENT IN CoaL Seams—That the main
chemical change in coal formation is carbon enrichment by
gradual elimination of hydrogen and oxygen is shown by
the proportions of these constituents in the sequence from
wood to anthracite. This process is at first bio-chemical,
being controlled by living ferments in the wood and bacteria.
Bertrand and Renault considered that the bio-chemical
influence lasts much longer than the first stage. Coal for-
mation has been regarded as mainly dependent on bacteria ;
but the particles so identified appear to be specks of inor-
ganic matter. The biochemical processes stop at an early
! The view that all coal is deposited under water has been recently
readvanced by Dr. Murray Stewart. He regards coal as due to bacterial
action in swamps and lagoons which converts vegetable matter into
particles of coal; they are washed into lakes or the sea and there deposited
as a coal mud, mixed with tree-stems which he regards as also floated
to their present positions, their erect position being due to the roots being
weighted and therefore sinking first (Geology of Oil Shale and Coal,
1926, pp. 19-21).
