106 



BRITISH EOCENE ELOUA. 



feet in diameter, has been enveloped as it stood, in one of the underlying lava-beds. 

 Owing evidently to its solidity and substantial girth, it resisted erect the fiery embrace 

 of a torrent of molten and liquid lava forty feet in depth, for the outer layers are still 

 preserved ; but subsequent prolonged exposure to damp and wet, acting from above, slowly 

 rotted the heart-wood, until little of the trunk remained beyond a cylinder or well 

 choked with rubble. A little further on lies the limb of a great tree, which, long before 

 the Ardtun river flowed in this direction, had helped with other debris to fill in a fissure. 



The examination I have made of the area surrounding the Ardtun leaf-beds, has 

 convinced me that these latter are stratigraphically much nearer the base of the Basalts 

 than the corresponding plant-beds of Antrim. Having regard to the greater thickness 

 of the series in this part of Scotland, and especially to the presence of extensive ash-beds 

 at its base, it is difficult to come to any conclusion other than that the Ardtun beds are 

 the oldest ; and the opinioH I had formed as to the relatively greater antiquity of the 

 Irish plants must thus be accepted with reserve. As far as its bearing on geology goes, 

 my work on fossil plants is for the present, and probably must be for some time to 

 come, destructive rather than constructive.^ Stratigraphical and physical evidence helps 

 us but little in these vast volcanic regions, and nothing but the most patient investigations 

 will ever bring the debris of plants, so wonderfully preserved in them, into line with other 

 organic fossils and make them acceptable to the geologist as trustworthy indications of age. 

 Our greatest hope of success lies in continually checking them off by the fossil floras of 

 the South of England, where the entire series of beds which make up the great group or 

 formation called the Eocene is displayed in a compass that not only renders their study 

 easy, but makes it certain that they are not separated from each other by very great 

 intervals of time. The age of each of the plant-beds included in this series is as absolutely 

 known as any fact of geology can be, for it is checked in every case by the intercalation 

 of deposits, teeming with marine and estuarine Mollusca, which are acknowledged by 

 everyone to be conclusive proofs of age. The discovery in the lowest of these beds of 

 types of plants which, until now, have been believed universally to occur only in the 

 Miocene would alone have rendered a complete revision of the whole subject necessary. 



The explorations and quarrying operations in the Ardtun beds were carried out by 

 means of a grant from the Royal Society. 



^ I am able to state that certain plants are not peculiar to the stages they are believed to characterise, 

 and hence that the ages of many plant deposits have been determined on erroneous premises. I have 

 reason to believe that other plants are absolutely confined in some areas to beds of very different age to 

 what has been hitherto assumed, and that many plants from different areas, believed to be identical, are 

 really not so. But I cannot yet claim that fossil Floras ofiFer evidence of age of the same value as fossil 

 Faunas. 



