A PIECE OF COAL. l8l 
the top. Their stems make up a great deal of the coal, 
and the bark of their trunks is often found in the clays 
above, squeezed flat in lengths of 30, 60, or 70 feet. 
Sometimes, instead of being flat the bark is still in the 
shape of a trunk, and the interior is filled with sand ; 
and then the trunk is very heavy, and if the miners do 
not prop the roof up well it falls down and kills those 
beneath it. Stigmaria (Fig. 48, page 175) is the root of 
the Sigillaria, and is found in the clays below the coal. 
Botanists are not yet quite certain about the seed- 
cases of this tree, but Fig 
Mr. Carruthers believes 
that they grew inside 
the base of the leaves, as 
they do in the quillwort, 
a small plant which 
grows at the bottom of 
our mountain lakes. 
But what is that 
curious reed-like stem 
we found in the piece 
of shale (see Fig. 47) ? 
That stem is very im- 
portant, for it belonged 
to a plant called a Cala* 
mite, which, as we shall 
see presently, helped to 
sift the earth away from 
the coal and keep it Equisetum or horsetail, 
pure. This plant was 
a near relation of the "horsetail," or Equisetum, which 
grows in our marshes ; only, just as in the case of the 
other trees, it was enormously larger, being often 
