Kin ah an — On Irish Arenaceous Rocks. 515 



offices of one company through a large range of country will all be built of one stone, 

 while in all those belonging to another company a different stone is used ; the stones 

 often being brought from a distance, although better stones could be procured in the 

 vicinity.] 



GEOLOGICAL EPITOME. 



The Arenaceous rocks range all through the different geo- 

 logical groups. It is therefore expedient, before entering into 

 detail, to give an epitome of the present state of Irish Geology. 

 In this the classification of the groups is that adopted in the 

 Table of Strata in the first Paper of this series on Irish Economic 

 Geology (ante, "Metal Mining," p. 204). 



CAMBRIAN AND ARENIG. 



[These groups are so mixed up as to necessitate their being described together.] 



From the latest fossil evidence brought before the public, which 

 is, that supposed Arenig type fossils have been found in the gneiss 

 and schist series at Fintown, it would appear that it is now incon- 

 testably proved that the oldest rock in Donegal cannot be more 

 ancient than Cambrian. Consequently, all the other similar rocks 

 in Ireland, which some have called Laurentian, are probably of the 

 same age or younger : that is, these metamorphosed rocks must be 

 the equivalents of either the Ordovician, Arenig, or Cambrian. 



In 1862 Jukes, and in 1863 Sterry Hunt (after Laurentian 

 rocks were proved to exist in Scotland), suggested the possible 

 Laurentian age of the Donegal gneiss. In 1865 Murchison an- 

 nounced the existence of Laurentian rocks in the Twelve Pins 

 {Bennabeola), Connemara, Co. Gal way ; but immediately after- 

 wards he withdrew this statement. In the Geology of Ireland 

 (1878), and subsequently in various Papers read before the Royal 

 Irish Academy, the Royal Dublin Society, and the Royal Geo- 

 logical Society of Ireland, I pointed out that some of the meta- 

 morphic rocks of Donegal, Antrim, Tyrone, Leitrim, Sligo, and 

 Mayo were probably Cambrians, but possibly Laurentians ; while 

 Dr. Hicks immediately afterwards suggested that the rocks called 



SCIEN. PIIOC. ll.D.S. — VOL. V. PT. VI. 2 N 



