140 STRATIGRAPHICAL GEOLOGY 



replaced by the Doolough green slates and grits which contain the 

 same graptolites. They pass up into the Mweelrea grits red and 

 green felspathic grits which in the lower 3000 feet include bands 

 of shale yielding Ogygia Buchi and brachiopods, but no graptolites ; 

 they are evidently of Llandilian age. In conformable succession 

 is an enormous and monotonous series of reddish felspathic grits, 

 without a single parting of shale and quite destitute of fossils, but 

 they are probably of Bala age. Thus we have the following 

 correlation : 



Feet. 



Bala / Mweelrea /Upper 9,000 



LlandihH Grits ( Lower 3,000 



T1 . fLeenane Grits and^ 



Llanvirn ^. , o,, , .... 2.500 



, J Doolough Slates j 

 ana ^ K Bencratf shales 60 



3 t Lower beds not seen . . .at least 500 



15,000 



To the eastward in the Glensaul and Tourmakeady districts a 

 much greater thickness of Arenig Beds is exposed, and they 

 consist partly of coarse grits and conglomerates, partly of a thick 

 mass of volcanic materials, which lower include some fossiliferous 

 beds of limestone and shale. Omitting the felsitic lavas, most of 

 which are intrusive, the succession described by Messrs. Gardiner 

 and Reynolds in this area may be summarised as follows : 23 



Feet. 



Coarse conglomerate and sandstone (very thick) 



Calcareous tuffs, with bands of limestone . . . . . ? 300 

 Massive agglomerate of felsite-fragments ..... 750 

 Volcanic tuffs with a shale yielding Didymograj^tus hirundo and 



D. gibberulus ......... 150 



Coarse felspathic grits and tuffs . . . . . . .700 



Coarse quartzose and some felspathic grits ..... 150 



Fine siliceous grits including a black shale with Tctragrapti and 



cherts containing Eadiolaria ....... 150 



Coarse grits and conglomerates .... seen for over 800 



3000 



It will be seen that the base of the Arenig Series is not exposed, 

 but as the lowest conglomerates contain pebbles of gneiss, mica- 

 schist, and hard grit they probably rest upon Archaean rocks. 



The lowest fossiliferous band has yielded five species of Tetra- 

 graptus, associated with Didymograptus extensiis, D. nanus, and 

 small forms of D. bifidus. The only other species found at the 

 higher horizon is Diplograptus dentatiis, and the fauna might be 

 Arenig or Llanvirn. The limestones contain a peculiar trilobite 

 fauna, which leads Mr. Cowper Reed to correlate them with the 



