DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 93 



layer, and retaining almost the colours of dead vegetation. Among the plants represented 

 is a large purple Ginkgo and a fine Platanites, one leaf measuring 15^ inches long by 

 10^ broad. The characteristic dicotyledonous leaves at this locality possessed relatively 

 large foliage.* 



To the early observations of Macculloch we are indebted for the record of an 

 interesting fact in connection with the vegetation of the land-surface over which the first 

 lava-flows spread. He figured a vertical tree trunk, imbedded in prismatic basalt, and 

 rightly referred it to some species of fir.t This relic may still be seen under the basalt- 

 precipices of Gribon. Mr Gardner found it to be "a large trunk of a coniferous tree, 

 five feet in diameter, perhaps Podocarpus, which has been enveloped, as it stood, in one 

 of the flows of trap to the height of 40 feet. Its solidity and girth evidently enabled it 

 to resist the fire, but it had decayed before the next flow passed over it, for its trunk is 

 a hollow cylinder filled with debris, and lined with the charred wood. A limb of another, 

 or perhaps the same tree, is in a fissure not far off." \ 



At different levels in the volcanic series of Mull, beds of lignite and even true coal 

 are observable. These seem to be always mere lenticular patches, only a few square yards 

 in extent. The best example I have met with is among the basalts near Carsaig. It is 

 in part a black glossy coal, and partly dull and shaly. Some years ago it was between 

 two and three feet thick, but now, owing to its having been dug away by the shepherds, 

 only some six or eight inches are to be seen. It lies between two basalt-flows, and 

 rapidly disappears on either side. 



More frequent than these inconstant layers of fossil vegetation are the thin partings 

 of tuff and layers of red clay, sometimes containing iron-ore, which occur at intervals 

 throughout the series between different flows of basalt. But even such intercalations are 

 of trifling thickness, and only of limited extent. The magnificent precipices of M'Gorry's 

 Head and Gribon expose a succession of beds of columnar amorphous and amygdaloidal 

 basalt, which must attain a thickness of at least 2500 feet, before they are overlain by the 

 higher group of lavas in Ben More. On the east side of the island, thin tuffs and bands 

 of basalt-conglomerate occur on different horizons among the bedded basalts, from near 

 the sea-level up to the summit of the ridge which culminates in Beinn Meadhon (2087 

 feet), Dun-da-Ghaoithe (2512 feet), and Mainnir-nam-Fiadh (2483 feet). 



Above the ordinary compact and amygdaloidal basalt comes the higher pale group 

 already referred to as forming the uppermost part of Ben More, whence it stretches 

 continuously along the pointed ridge of A'Chioch, and thence northwards into Beinn 

 Fhada. The same felspathic lavas are likewise found in two outliers, capping Beinn a' 

 Chraig, a mile further north, and I have found fragments of them on some of the loftier 

 ridges to the south-east. This highest and youngest group of lavas in the plateaux has 

 been reduced to mere isolated patches, and a little further denudation will remove it 



* For fuller local details regarding the Ardtun leaf-beds, I may refer to the original paper by the Duke of Argyll 

 {Quart. Jour. Geol. Soc., vii. p. 89), and to the recent memoir by Mr Starkie-Gardner (op. cit., xliii., 1887, p. 270). 

 t Western Islands, vol. i. p. 568, and plate xxi. fig. 1. % Quart. Jour. Geol. Soc, xliii. p. 283. 



VOL. XXXV. PART 2. N 



