Lower Silurian 



^ 7 lie Irish yatjij-alist. January, 



111 1893-4 the district was revised for the Geological Survey 

 In- Mr. Kilroe. Save that he correlated the Leeiiane Grits 

 with those of Rossroe, and removed the lower third of the 

 Mweelrea Beds to the Doolough Series, the old grouping was 

 retained by him, but important alterations were made regard- 

 ing the geological i^ge of the various divisions. 



The table of strata, as revised b}' him,^ is given below. 

 c Salrock Slates (Ludlow) 

 Upper Silurian \ Owenduff Series (Wenlock) 



I Mweelrea Grits (restricted) (Llandovery) 

 Doolough Series (Bala) 



Leenane Grits and Rossroe Grits (Llandeilo) 

 , Black Slate at Bencraff (Arenig). 



An important discovery of a band of black slates and cherts 

 at the base of the Leenane grits was made by him. These 

 yielded graptolites regarded as of probably Arenig age. 



We agree with the separation of these divisions according to 

 lithological characters, but the palaeontological evidence on 

 which the several ages of the rock-groups was originally deter- 

 mined was in more than one case too scanty to justify the 

 inferences drawn. This, and the incorrect determination of 

 fOvSsils, appears to have misled the field-men, and been the 

 chief cause of the anomalies in the published map, point'.dout 

 by Mr. Kilroe in the paper already referred to. 



Structurally the strata are arranged in a gentle east and 

 west syncline, with its axis running a short distance north of 

 Killary Harbour. The highest beds, however, do not lie in 

 the centre of the syncline, as a great reversed fault runs along 

 the south side of the Harbour, hading to the north, and bring- 

 ing up Ordovician rocks on to the Silurian. The axis of the 

 main syncline is shifted at the east end of Killary Harbour a 

 mile or more to the south by an important wrench-fault, 

 coming from the Maam valley and running in a north-westerly 

 direction. 



All the rocks are more or less cleaved, but the folding is on 

 the whole quite gentle, although in the northern part of the 

 district the Doolough Slates show a considerable compression 

 and packing from the north against the great mass of the 



' rroi. Royal Irish Aiadcniy, vol. xxvi , sect. B., pp. 129-160, 1907. 



