Palceozoic Rocks of Gahvay and elsewhere in Ireland. 351 



of the underlying older quartzites, and not northward, as repre- 

 sented in these sections.* 



In the Transactions of this Society, vol. i., pi. xxi., fig. 6, Dr. Hull 

 gives a nearly north and south section across West Galway, in 

 which the supposed Lauren tian rocks are separated from the others 

 by the great fault of the Clifden and Oughterard valley. This fault, 

 as I have shown in the Government Survey Memoir, is a down- 

 throw to the south. To meet Dr. Hull's idea, it would have to 

 be a great downthrow to the north of over 5,300 feet. Even if 

 Dr. Hull's general dip were correct, I do not understand how 

 the rocks could get into the position represented as the result of 

 a northern downthrow. 



As has already been mentioned. Dr. King, before I examined 

 tne country, suggested that the quartzites, ophiolites, &c., of the 

 Bennabeola and neighbouring hill groups were Cambrian or 

 older ; while, during the time of my examination, Sir E. I. Murchi- 

 son, in a paper in the Geological Magazine, gave it as his opinion 

 that the rocks of the Bennabeola mountains were of Laurentian 

 age ; which opinion he withdrew in a paper in the next number 

 of that journal. During my explorations I searched diligently 

 for an unconformability, or some indications of one, but found 

 none ; and my examination of the district, with a due and deli- 

 berate consideration of the facts detected, forced me to come to 

 the conclusion that the rocks formed a continuous sequence, the 

 oldest being of Cambrian and the youngest of Cambro-Silurian 

 ages, with between them rocks representing the " Arenig rocks " 

 of Wales, on the passage-beds between the Cambrians and the 

 Cambro-Silurians ; and from his preface to the Geological Sur- 

 vey Memoir (Exp. sheets 93, &c.), it appears that Professor A. C. 

 Ramsay came to a somewhat similar conclusion during a short run 

 through the country in the company of Dr. HuU. 



As with the Tyrone rocks, so with these — the changes in opinion 

 as to their supposed age have been sudden and uuexplainable. In 



* The maps and, sections were carefully explained to Sir Richard Griffith, and left with 

 him to examine, who, when returning them to me, expressed in a letter that he considered 

 I had unravelled the geology of the district ; and I find by a subsequent letter, dated 

 Hendersyde Park, Kelso, KB., Nov. 19, 1872, that Griffith was of the opinion that the 

 rocks of the Forth mountains, Co. Wexford, the Cambrians of Wicklow, the rocks of the 

 Twelve Pins, Counemara, and the rocks of Donegal, probably all belonged to one and the 

 same geological period. 



SciEN. Puoc. R.D.S., Vol. hi.. Ft. vi. 2 E 2 



