84 DR 'GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



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 some- 

 times 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 more recently been studied and described by Heer, Mr Baily, and Mr Starkie 

 Gardner.* 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 paper, 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 he 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 embedded plants definitely limit them to 

 the earlier half of Tertiary time. 



§ 2. Areas of the Plateaux and Succession of Eocks in them. 



There are four districts in which the original widespread lava-fields have been less 

 extensively eroded than elsewhere, or at least where they have survived in larger and 

 thicker masses. Whether or not each of them was an isolated area of volcanic activity 

 cannot now be determined. Their several outflows of lava may have united into one 

 continuous volcanic tract, and their present isolation may be due entirely to subterranean 

 movements and denudation. There is a certain convenience, however, in treating 

 them separately. They are — 1. Antrim; 2. Mull; 3. Small Isles ; 4. Skye. To these 

 might be added the Shiant Isles and St Kilda. 



1. AntrimA — The largest of the basalt-plateaux of Britain is that which forms so 

 prominent a feature in the scenery and geology of the north of Ireland, stretching from 

 Lough Foyle to Belfast Lough, and from Bathlin Island to beyond the southern margin 

 of Lough Neagh. Its area may be roughly computed at about 2000 square miles. But, 



* Mr Gardner is now describing and illustrating the flora fully for the Palasontographical Society; see vols, 

 xxxviii., xxxix. et seq. 



t The basalts of Antrim are the subject of an abundant literature. I may refer particularly to the papers of 

 Berger and Conybeare (Trans. Geol. Soc, iii.), the Geological Report of Portlock, and the Explanations of the Sheets 

 of the Geological Survey of Ireland. 



