242 LEAF-BEDS OF MULL IN SCOTLAND. [Ch. XIY. 



The croziers of some of the young ferns are very perfect, and were 

 at first mistaken by collectors for shells of the genus Planorbis. On 

 the whole, the vegetation of Bovey implies the existence in Devonshire, 

 in the Lower Miocene period, of a sub-tropical climate. 



Scotland. — Isle of Mull. — In the sea-cliffs forming the headland of 

 Ardtun on the west coast of Mull, in the Hebrides, several bands of 

 tertiary strata containing leaves of dicotyledonous plants were discov- 

 ered in 1851 by the Duke of Argyle.* From his description it ap- 

 pears that there are three leaf-beds, varying in thickness from 1-J to 

 2|- feet, which are interstratified with volcanic tuff and trap, the whole 

 mass being about 130 feet in thickness. A sheet of; basalt 40 feet 

 thick covers the whole ; and another columnar bed of the same rock, 

 10 feet thick, is exposed at the bottom of the cliff. One of the leaf- 

 beds consists of a compressed mass of leaves unaccompanied by any 

 stems, as if they had been blown into a marsh where a species of 

 Equisetum grew, of which the remains are plentifully embedded in 

 clay. 



It is supposed by the Duke of Argyle that this formation was ac- 

 cumulated in a shallow lake or marsh in the neighborhood of a vol- 

 cano, which emitted showers of ashes and streams of lava. The 

 tufaceous envelope of the fossils may have fallen into the lake from the 

 air as volcanic dust, or have been washed down. into it as mud from 

 the adjoining land. Even without the aid of organic remains we 

 might have decided that the deposit was newer than the chalk, for 

 chalk flints containing cretaceous fossils were detected by the Duke in 

 the principal mass of volcanic ashes or tuff, j 



The late Edward Forbes observed that some of the plants of this 

 formation resembled those of Croatia, described by Unger, and his 

 opinion has been confirmed by Professor Heer, who found that the 

 conifer most prevalent was the Sequoia Lang sdorfii (figs. 201, 202), 

 also Corylus grosse-dentata, a Lower Miocene species of Switzerland 

 .and of Menat in Auvergne. There is likewise a plane tree, the leaves 

 of which seem to agree with those of Platanus aceroides (fig. 187, p. 

 254), and a fern which is as yet peculiar to Mull, Filiates hehridica, 

 Forbes. 



These interesting discoveries in Mull naturally raise the question, 

 whether the basalt of Antrim in Ireland, and of the celebrated Giant's 

 Causeway, may not be of the same age. For in Antrim the basalt 

 overlies the chalk, and the upper mass of it covers everywhere a bed 

 of lignite and charcoal, in which wood, with the fibre well preserved, 

 and evidently dicotyledonous, is enclosed. The general dearth of 

 strata in the British Isles, intermediate in age between the formation 

 of the Eocene and Pliocene periods, may arise, says Professor Forbes, 

 from the extent of dry land which prevailed in that vast interval of 

 time. If land predominated, the only monuments we are likely ever 



* Quart. Geol. Journ., 1851, p. 89. f Ibid., p. 90. 



