Page:A Treatise on Geology, volume 1.djvu/186

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170
CHAP. VI.
A TREATISE ON GEOLOGY.

shire (Dudley), it does not exceed 1000 feet. The most variable parts, in all coal tracts, are the sandstones and shales; the most regular parts are the coal beds and ironstones.

Organic Remains.—The forms of life buried in the carboniferous system of strata are exceedingly numerous and varied, and, being generally in an excellent state of preservation, allow of a most strict comparison with existing types. They consist of very many races of plants, abundance of zoophyta, multitudes of mollusca, some Crustacea, many fishes, but, as far as we yet know, neither reptiles, birds, nor mammalia. Many of the plants, indeed by far the greater number, are of terrestrial growth: all the zoophyta, and nearly all the mollusca, Crustacea, and fishes, are marine. The excepted mollusca occur among the remains of plants swept down from the land: the excepted Crustacea are those referred to by Dr. Hibbert, in his account of the Burdiehouse limestones, with which also a few fishes are found, which, by this author, are referred to a freshwater origin.

The plants are partly very similar to existing races, as the large group of ferns generally, and partly appear altogether unlike them, as the large-furrowed stems of sigillaria, the quincuncially ornamented stigmaria, &c. On making the most close comparison which the subject admits, we find that among the fossil ferns are arborescent species, to which we can only find parallels in warm or else Australian regions; that the same analogy to the productions of a warm climate is suggested by fossil equiseta, and confirmed by the lepidodendra, which seem related to existing lycopodiaceæ in structure, though enormously surpassing them in dimensions. Even the sigillariæ, when carefully studied, though they be not cacti, nor euphorbias, nor arborescent ferns, are so much like those singular plants of hot climates, as to add considerably to the accumulating evidence in this direction.

The following is a brief summary of the plants:—