398 
ELEMENTS OF GEOLOGY. 
Wales as an essential accompaniment of each of the eighty 
or more seams of coal met with in their coal-field. They 
are said to form thej^oor on which the coal rests; and some 
of them have a slight admixture of carbonaceous matter, 
while others are quite blackened by it. 
All of them, as Sir William Logan pointed out, are charac¬ 
terized by inclosing a peculiar species of fossil vegetable call¬ 
ed Stigmaria^ to the exclusion of other plants. It was also 
observed that, while in the overlying shales, or “roof” of the 
coal, ferns and trunks of trees abound without any Stigmarice^ 
and are flattened and compressed, those singular plants of 
the underclay most commonly retain their natural forms, un¬ 
flattened and branching freely, and sending out their slender 
rootlets, formerly thought to be leaves, through the mud in 
all directions. Several species of Stigmaria long been 
known to botanists, and described by them, before their po¬ 
sition under each seam of coal was pointed out, and before 
their true nature as the roots of trees (some having been act¬ 
ually found attached to the base of Sigillaria stumps) was 
recognized. It was conjectured that they might be aquatic, 
perhaps floating plants, which sometimes extended their 
branches and leaves freely in fluid mud, in which they were 
finally enveloped. 
Now that all agree that these underclays are ancient soils, 
it follows that in every instance where w^e find them they at¬ 
test the terrestrial nature of the plants which formed the 
overlying coal, which consists of the trunks, branches, and 
leaves of the same plants. The trunks have generally fallen 
prostrate in the coal, but some of them still remain at right 
angles to the ancient soils (see Fig. 440, p. 411). Professor 
Goppert, after examining the fossil vegetables of the coal¬ 
fields of Germany, has detected, in beds of pure coal, remains 
of plants of every family hitherto known to occur fossil in 
the carboniferous rocks. Many seams, he remarks, are rich 
in Sijgillarice^ Lepidodendra^ and Stigmarice, the latter in 
such abundance as to appear to form the bulk of the coal. 
In some places, almost all the plants were calamites, in oth¬ 
ers ferns.^" 
Between the years 1837 and 1840, six fossil trees were dis¬ 
covered in the coal-fields of Lancashire, where it is intersect¬ 
ed by the Bolton railway. They were all at right angles to 
the plane of the bed, which dips about 15° to the south. 
The distance between the first and the last was more than 
100 feet, and the roots of all were imbedded in a soft argil¬ 
laceous shale. In the same plane with the roots is a bed of 
* Quart. Geol. Journ., vol. v., Mem., p. 17. 
