Chapter xxxviii

.

The vegetable matter has in some places gathered into lenticular seams of lignite, and even occasionally of black glossy coal. Amber also has been found in the lignite. Where the vegetation has been exposed to the

## action of intrusive dykes or sheets, it has sometimes passed into the

state of graphite.

The remarkable terrestrial flora found in the leaf-beds, and in association with the lignites, was first made known by the descriptions of Edward Forbes already referred to, and has subsequently been studied and described by Heer, W. H. Baily, and Mr. Starkie Gardner.[229] It was regarded by Forbes as of Miocene age, and this view has generally been adopted by geologists. Mr. Starkie Gardner, however, contends that it indicates a much wider range of geological time. He believes that a succession of floras may be recognised, the oldest belonging to an early part of the Eocene period. Terrestrial plants, it must be admitted, are not always a reliable test of geological age, and I am not yet satisfied that in this instance they afford evidence of such a chronological sequence as Mr. Gardner claims, though I am convinced that the Tertiary volcanic period was long enough to have allowed of the development of considerable changes in the character of the vegetation.

[Footnote 229: On this subject consult Duke of Argyll, _Quart. Journ. Geol. Soc._ vol. vii. (1851), p. 89; E. Forbes, _Ibid._ p. 103; W. H. Baily, _op. cit._ xxv. (1869), pp. 162, 357; _Brit. Assoc. Rep._ (1879) p. 162; (1880) p. 107; (1881) p. 151; (1884) p. 209; Mr. J. Starkie Gardner, _Palæontographical Society_, vols. xxxviii. xxxix. In the last of Mr. Baily's papers he notices that "the Rev. Dr. Grainger found a portion of a fish (_Percidæ_, possibly _Lates_)." The discovery of the remains of a fresh-water fish is an important additional testimony to the terrestrial conditions under which the lavas were erupted. The genus _Lates_ now inhabits the Nile and the Ganges.]

For the purpose of the present volume, however, the precise stage in the geological record, which this flora indicates, is of less consequence than the broad fact that the plants prove beyond all question that the basalts among which they lie were erupted on land during the older part of the long succession of Tertiary periods. Their value in this respect cannot be overestimated. Stratigraphical evidence shows that the eruptions must be later than the Upper Chalk; but the imbedded plants definitely limit them to the earlier half of Tertiary time.

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