28 Scientific Proceedings, Royal Dublin Society. 



hornhlendite series; while to the south-west quartzites were up- 

 thrust on to it. From Knockanteenbeg, for over nine miles towards 

 the south-west, the series of limestones and hornhlendite crosses 

 the country independently, and it is not found in connexion with 

 the Great quartzite until Lough Finn is reached, and here it is 

 evident that the latter has been upthrust on to the former. From 

 what has been shown it is evident that wherever the Great 

 quartzite comes in contact with the Limestone hornhlendite series it 

 is purely by accident.^ 



The statement that the Cranford sericitic series and the Lime- 

 stone hornhlendite series are one and the same group seems to me 

 to be quite indefensible. Even if the two groups were not 

 so essentially different, their positions, relatively to the Great 

 quartzite, illustrate the untenability of the statement. Every- 

 where the Cranford sericitic series lies in a regular sequence on the 

 quartzite, while usually the Limestone hornhlendite series is absent, 

 and in no place does it occur in a regular sequence under the 

 quartzite. 



How, then, could a group that always has a regular position 

 in regard to the Great quartzite be identical with one that has an 

 abnormal position, such as is given to it by the supporters of the 

 theory that its present position depends on upthrusting or other 

 faults ? 



Probable Ages of the Mocks. 



In the " Geology of Ireland " (1878) it is suggested that these 

 Donegal rocks are probably metamorphosed Ordovicians and Cam- 

 brians ; while in a more recent publication (" Palaeozoic Rocks o£ 

 Galway and elsewhere in Ireland, said to be Laurentiaus " — Scient. 

 Proc. Roy. Dub. Sac, vol. iii., n. s., p. 347, 1882) it is shown that the 

 metamorphic rocks in South Donegal, to the westward of Pettigo, 

 must be of the same age as the metamorphic rocks of the Pomeroy 

 district, Co. Tyrone, and therefore possibly pre-Cambrian. In a 

 still more recent publication (" The Economic Geology of Ireland," 

 Roy. Geol. Soc. Jr. Proc, vol. xviii.) I state that there are 



^ These details, in connexion with the rocks in the neighbourhood of Gartan Lakes^ 

 are necessary, as all account of them has been cut out of the Geological Survey 

 Memoirs. 



