428 G. H. Kinahan — Possible Irish Laurentian Hocks. 



the newer rocks are more metamorphosed than the older, and to me 

 it would appear that it is the newer rocks that are now suggested 

 to be Pre-Cambrian. The older rocks in previous publications I 

 have suggested to be Upper Cambrians (Arenig rocks). The rocks 

 supposed to be Upper Cambrians seem to extend north-east across 

 Sligo, into Leitrim, near Manor Hamilton ; possibly they also occur 

 between Charlestown and Ballaghaderreen in N.E. Mayo. The latter 

 small exposure was considered by Griffith and Jukes to be of old 

 rocks, although now they have been joined on to the associated 

 fossiliferous Silurians (Lower Old Red Sandstone). Such a classifi- 

 cation, however, must be petrologically incorrect ; as these older 

 rocks were upturned, contorted, and considerably metamorphosed 

 and denuded, prior to the Silurians being deposited on them. This 

 is proved hj the younger rocks being made up of the detritus of 

 the older ones. These metamorphic rocks, as I have pointed out in 

 a paper piiblished by the Eoyal Irish Academy, although litho- 

 logically very similar to the lower zone (Arenig rocks) of West 

 Galway, are also like those of the upper zone, and possibly may be 

 of the latter age (Llandeilo). 



The gneissose rocks of Erris, N.W. Mayo, are a remarkable group, 

 Griffith and the different other early geologists pointed out that they 

 seemed to be much older than the associated metamorphic rocks. 

 Microscopically they seem to be identical with the rocks of Carnsore, 

 and for this and other reasons, mentioned in a paper read before 

 the Eoyal Geological Society of Ireland, I suggested that they were of 

 Cambrian age. All that can be positively asserted about them is, 

 they are older than the associated metamorphic rocks. But what 

 is the age of the latter ? They are said to be of Cambro-Silurian age ; 

 yet in composition, aspect, etc., they are very similar to the rocks 

 south of Louisburgh, in the same county, that have been proved by 

 Symes to be Silurians. Their position only, therefore, does not prove 

 them to be older than Cambro-Silurian. 



In Donegal and the adjoining parts of Derry and Tyrone there is 

 a large tract which was formerly supposed to be occupied solely by 

 metamorphic and granitic rocks. But of late years Dr. King, of 

 Galway, has discovered in Kilmaghrenan impressions that seem to 

 be of fossils of Cambro-Silurian age ; this however is not as yet 

 positively proved, as some authorities deny that they are fossils. 

 In the S.E. of this area (Co. Tyrone), as I have proved in a 

 paper published by the Royal Irish Academy, there ai^e very old 

 rocks to the north of Pomeroy, while to the N.W. there are also 

 very old rocks, which years ago were suggested by Jukes to be 

 of Laurentian age, on account of their likeness to the Scotch 

 Laurentians. Now, however, if King is correct, it is possible they 

 may be only Cambrians, overlaid by Cambro-Silurian ; the latter 

 in part highly metamorphic, as is tho case elsewhere. 



It must, however, be allowed that nowhere in Ireland are there 

 rocks lithologically similar to those of Donegal ; the nearest ap- 

 proach to them being some of the metamorphic rocks of N.W. 

 Galway ; but the latter stratigraphically are Cambro-Siluriaus 



