82 J. S. GAEDNEB ON THE LOWER EOCElirE PLANT-BEDS 



12. On the Lo"web EocEifE Plant-beds of the Basaltic Poemation of 

 Flstee. By J. Siaeele Gaednee, Esq., F.L.S., F.Gr.S. (Eead 

 December 3, 1884.) 



Tke, general features of the basaltic district of Ireland have been too 

 frequently described to need more than a few introductory words 

 here *. It is situated in the jS".E. of Ireland, and forms a plateau 

 with steep escarpments on every side, except in the direction of 

 Lough iS'eagh. The formation rests upon a very uneven surface, and 

 the thickness of the lowest of the three divisions into which it has 

 been classified is thus very variable. It is described by Hull as 

 "silicated felspathic trachytes, porphyry, pearlstone, pitchstone." 

 But a good deal of the lava resting upon the Chalk appears outwardly 

 to be amorphous trap. The second division contains all the plant- 

 beds hitherto found in the basaltic formation in Ireland, with the 

 possible exception of those of Lough x^eagh. The third is composed 

 of solid sheets of columnar and amorphous basalt. The greatest 

 total thickness observed in Ireland is at Sleamish, a mountain 1437 

 feet high, aU that is visible (at least 1100 or 1200 feet) being com- 

 posed entirely of basalt. These basalts have been eroded on a colossal 

 scale, for the valleys are scooped out of solid horizontal sheets, as 

 pointed out by Conybeare to this Society so long ago as 1816 f. 

 They seem to have formed almost the southern limit of a formation 

 which once stretched continuously to Iceland, and to what thickness 

 they were origtaally erupted can never be known ; but the upper or 

 columnar basalt series, now only 400 or 500 feet thick in Antrim, is 

 believed to expand to the immense total of from 3000 to 4000 feet in 

 Mull, only some 70 miles distant. One of their most characteristic 

 features, as in Iceland, is the relative rarity of dykes. TThen present 

 their prevailing trend seems to be S.E. to ]S'.W. I believe the idea 

 that any traces of necks or craters remain, through which such masses 

 could have been erupted, though once strongly advocated J, is now 

 abandoned, and the theory that at least the upper series welled up 

 through fissures, represented in part by the four gigantic dykes seen 

 on the north coast, will, perhaps, be more generally acceptable. 



The middle horizon, in which most, if not all, of the sedimentary 

 deposits containing plants occur, marks a very considerable interval 

 of time during which the slow disintegration of the basalts permitted 

 the formation of iron-ores to a thickness of 40 or 50 feet and also of 

 great masses of lignite. The sedimentary deposits of Glenarm and 

 of BaUypalady, at least, show the passage of larger bodies of swiftly- 

 flowing water, and were formed in the bed of a river. The lignites 



* Hull, Phys. Geol. & Geogr. Ireland, chapter iii. ; Einahan, Geology of Ire- 

 land, 1878 ; Portlock's Eeport on Londonderry and Tyrone ; &c. 



t Geol. Trans, vol. iii ; Berger on the Geological Features of North-east 

 Ireland, p. 127. 



X Hull, Phys. Geol. & Geogr. Ireland, 1878, chapter iii. 



