COAL 263 
Lignite or Coal. On this subject, Mr. Archibald Geilie 
says: “An example of this alteration was observed a few 
years ago in the Dorothea Mine, Clausthal.’ Some of the 
timber, in a long disused level, filled with slate-rubbish and 
saturated with mine-water from decomposing pyrites, was 
found to have a leathery consistence when wet, but on 
exposure to the air hardened to firm and ordinary brown Coal 
with the typical brown colour and external fibrous structure, 
and having the internal fracture of a black glossy pitch 
Coal. This change must have been produced within less 
than four centuries—the time since the levels were opened.” 
The vegetation forming or giving rise to Coal deposits, 
appears to have grown in wide shallow lagoons bordering 
the sea, or on low swamps and flood-plains near the 
mouths of rivers. In these situations dense masses of 
vegetation, differing considerably from that which grows 
in the same situations at the present time, clothed the 
moist soil. Over this the sea gradually found its way 
as the land subsided, burying the vegetation under thick 
layers of sand and mud. Removed from the atmosphere 
and under this enormous pressure, accompanied perhaps by 
subterranean heat, the vegetable matter was transformed 
by slow chemical changes (principally the removal of oxygen 
and hydrogen from the original woody tissue) into the 
mineral Coal, which in chemical composition, differs from 
wood mainly in having a smaller proportion of oxygen 
and hydrogen, and being richer in carbon. Subsequent 
elevation of the formerly depressed area would raise the 
vegetation, now converted into a Coal seam, above the 
level of the sea, 
Mode of occurrence.—Coal occurs in layers or seams 
more or less horizontal in position, and ranging consider- 
ably in thickness. Below the seam is found almost in- 
variably a layer of fire-clay, so largely used in the 
