In other cases, as in the marbles
of Devonshire, the matrix is so compact and crystalline that
the included corals can only be satisfactorily studied by means
of polished sections. In other cases, again, the corals have
been more or less completely converted into flint, as in the
Corniferous limestone of North America. When this is the case,
they often come, by the action of the weather, to stand out from
the enclosing rock in the boldest relief, exhibiting to the observer
the most minute details of their organization. As before, the
principal representatives of the Corals are still referable to
the groups of the _Rugosa_ and _Tabulata_. Amongst the Rugose
group we find a vast number of simple "cup-corals," generally
known by the quarrymen as "horns," from their shape. Of the many
forms of these, the species of _Cyathophyllum, Heliophyllum_
(fig. 82), _Zaphrentis_ (fig. 81), and _Cystiphyllum_ (fig. 80),
are perhaps those most abundantly represented--none of these
genera, however, except _Heliophyllum_, being peculiar to the
Devonian period. There are also numerous compound Rugose corals,
such as species of _Eridophyllum, Diphyphyllum, Syringopora,
Phillipsastroea_, and some of the forms of _Cyathophyllum_ and
_Crepidophyllum_ (fig. 83). Some of these compound corals attain
a very large size, and form of themselves regular beds, which
have an analogy, at any rate, with existing coral-reefs, though
there are grounds for believing that these ancient types differed
from the modern reef-builders in being inhabitants of deep water.
Pages:
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232