106 MR. GARDINER AND PROF. REYNOLDS ON THE [May IQOO. 



II. The Sedimentary Eocks and the Tuffs. 



As has been already mentioned, thick grits and coarse conglo- 

 merates bound the whole district on the west, and play also a 

 prominent part in its southern portion near Tourmakeady ; except 

 for them, however, the non-igneous rocks are but poorly exposed. 

 The district is noteworthy for the bad preservation and scanty 

 character of the fossils, a fact which adds greatly to the difficulty 

 of understanding; its structure. 



(a) The Arenig Rocks (Mount Partry Beds). 



Scattered throughout the district, especially near its eastern 

 border, are a number of exposures of conglomerate, coarse grit, 

 radiolarian chert, and slate, sometimes associated with interbedded 

 tuff. The grits and cherts are often mentioned in the Survey 

 Memoir, 1 the cherts, which are referred to as ' jasperized shales or 

 hornstones,' being recognized by the authors of the memoir, 2 and 

 by Mr. Kilroe & Sir Archibald Geikie, 3 as the oldest rocks in the 

 district. The coarse conglomerate was, however, regarded as of 

 much later date than the grits and cherts, being grouped, with that 

 exposed all along the western border of the area, as Llandovery by 

 the authors of the Survey Memoir, and as Bala by Sir Archibald 

 Geikie & Mr. Kilroe. Mr. Kilroe 4 has, however, recently come to 

 the conclusion that the apparent succession is the true one, and 

 that this eastern conglomerate is of the same general age as the 

 slates and cherts with which it is associated, while that exposed 

 along the western border is of far more recent date (Bala). 



The two conglomerates contrast somewhat strongly as regards 

 the nature of their pebbles : in that exposed along the western 

 border the pebbles are mainly of granite and felsite ; while in that 

 associated with the cherts and slates in the south-eastern part of 

 the area, though occasional pebbles of felsite and quartz and rather 

 more numerous pebbles of gneiss and mica-schist occur, grit-pebbles 

 vastly preponderate. This is one of the reasons which leads us to 

 accept Mr. Kilroe's conclusion as to the age of the conglomerate 

 associated with the slaty and cherty series. The resemblance of the 

 cherts to those of Arenig age in the Ballantrae district is extremely 

 close. 



The general dip of these beds is in a north-westerly or north- 

 north-westerly direction, which is the usual dip of most of the 

 rocks in this district, and there is frequent evidence of disturb- 

 ance near the exposures of cherts and slates. 



1 Explan. of Sheets 73 & 74 (in part), 83 & 84, Mem. Geol. Surv. Irel. 1876, 

 pp. 21, 61, 62, 65. 2 ibid. p. 21. 



3 Ann. Eep. Geol. Surv. TJ. K. for 1896 (1897) p. 49. 



4 Proc. Roy. Irish Acad. vol. xxvi, sect. B, no. 10 (1907) p. 130 & footnote 1. 



