66 Scientific Proceedings, Royal Dublin Society. 



contour. This would, according to Mr. "Wilkinson's estimate of the 

 thickness of the Toredale Series in that area, place the Millstone 

 Grit some 850 feet higher. Thus no substantial argument can be 

 deduced from such considerations in favour of the beds being repre- 

 sentative of Millstone Grit ; nor yet from their lithological similarity 

 to this member ; for, derived as they obviously have been from 

 quartzite, which once spread out widely over the adjoining area, 

 they might be expected to consist of white sandstone, &c. 



Passing by the Toredale Series, which comes next in succession 

 below the Millstone Grit, there is a possibility that the Slieve League 

 beds may represent the Calp or Middle Limestone Series, which 

 shows a considerable thickness of sandstone, and conglomerates at 

 Mount Charles and Lough Easke, near Donegal. But it is more 

 probable, from the considerations following, that they represent the 

 lowest member of the Formation ; and it is strong corroborative 

 evidence towards such a conclusion, that it was Lower Carboniferous 

 Sandstone, near Killybegs, which yielded Sir E. Griffith and Dr. 

 Haughton the specimens of Stigmaria already referred to. 



If the denuded portions of the Lower Carboniferous Sandstone 

 and conglomerate were restored, near Largy, six miles east of 

 Slieve League, this member would reach an elevation of 1000 feet 

 above datum, or some 700 feet lower than the point at which the 

 conglomerate beds repose on the older rocks of the hill. This may 

 be lessened by, perhaps, 600 feet at least, due to the occurrence of 

 two western up-throws along lines of post-Carboniferous faults, 

 which run south-westward between Slieve League and Largy. The 

 100 feet difference of level which remain distributed over the six 

 miles would give an inappreciable slope, compared with that which 

 is observable in passing from the basal bed on the north-western limb 

 of the hill to that on the central mass. This amounts to 200 feet 

 in less than a mile. 



It may be added that the occurrence of these beds at so high an 

 elevation in South Donegal implies a former extension of Carbon- 

 iferous strata, not only throughout the county, but westward, over 

 ground now covered by waters of the Atlantic ocean. With this 

 conclusion may also be inferred the vast extent to which denuda- 

 tion has obtained since the Carboniferous epoch. 



