Sometimes, instead of being
flat the bark is still in the shape of a trunk, and the interior
is filled with sane; 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 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 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 important, for it
belonged to a plant called a Calamite, which, as we shall see
presently, helped to sift the earth away from the coal and keep
it 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 20 feet
high, whereas the little Equisetum, Fig.
Pages:
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238