THE TERTIARY VOLCANOES 
BOOK VIII 
(d) There occur also grey and black clays and shales, of ordinary sedi- 
mentary materials, containing leaves of terrestrial plants (leaf-beds), with 
occasional wing-cases of beetles, sometimes associated with impure lime- 
stones, but more frequently with sandstones and indurated gravels or 
conglomerates containing pieces of fossil wood. These intercalated bands 
undoubtedly indicate the action of running water, sometimes even of river- 
floods, and the accumulation of sediment in hollows of the exposed flows of 
basalt at intervals during the piling up of the successive lava-sheets that 
form the plateaux. The alternation of fluviatile gravels with volcanic tuffs, 
fluviatile conglomerates, and lava-streams, is admirably displayed in the 
island of Canna, as will be narrated in detail in 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 . 1 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. 
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 over- 
estimated. 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. 
1 On this subject consult Duke of Argyll, Quart. Journ. Qeol. Son. vol. vii. (1851), p. 89; K. 
Forbes, Ibid. p. 103 ; W. H. Baily, op. cit. xxv. (1869), pp. 162, 357 ; Brit. Assoc. Hep. (1879) 
p. 162 ; (1880) p. 107 ; (1881) p. 151 ; (1884) p. 209 ; Mr. J. Starkie Gardner, Palccontuyraphical 
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 ( Pereidcc , possibly Bates').’’ 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 Laics now inhabits the Nile and the Gauges. 
