132 
STIGMARIA. 
sand, and distributing these matters in 
water, and afterwards the bed thus formed 
was elevated, vegetation resumed its reign 
to perish as its preceding exhibitions had 
done in the same place. 
A curious fact has been noticed re¬ 
specting the base of the layers of coal; it 
was first remarked by Mr. Logan in the 
Welch Coal field, and is now regarded as 
established, namely, that the underlying 
stratum is almost invariably a clay containing 
Stigmaria^ the presumed roots of the trees 
called Sigillaria. The clay bed varies in 
thickness from a few inches to ten feet, but 
is always found underneath the true coal, 
and contains these root-like plants and 
leaves. Long portions of root are connect¬ 
ed with the main stem, and are thickly 
matted and entangled amidst the clay. In 
the recent memoir of the Ordnance Survey 
the facts are thus stated. These under¬ 
beds are now^ known to be common in coal 
districts referrible to the Palaeozoic series; 
we have observed them beneath all the true 
coal beds we have seen in Great Britain and 
Ireland. Mr. Logan notices them in Ayr- 
