I909- CarpuTHERS <S: ^l\i¥V.--Pa/rco.::ou'RocksofK-i//arv. 9 



Mweelrea Grits. At the same time the slates show an appre- 

 ciable development of sericitic mica, and sometimes their 

 cleavage is crossed by a later strain-slip-cleavage. 



The oldest fossiliferous rocks of the district are of Arenig 

 age. Sonth of Killnry Harbour these comprise the Leenane 

 Grits, coarse conglomeratic grits, with very few shale partings, 

 the whole series being 2,500 feet thick. About 1,000 feet from 

 the base of the grits occurs a curious lenticular bed of brec- 

 ciated limestone, exposures of which are seen on Derryna- 

 clough Hill and near Dernasliggaun Lodge. Under the grits on 

 the southern flank of Bencraff is found a band of black shales 

 and cherts about 60 feet thick, which yield a rich graptolitic 

 fauna of Middle Arenig age. We found here four species of 

 Teh-agrapfns, Dichoo7-aptus octobracJiiatiis, Didymograptiis ex- 

 tensns and other forms, including two species of Diphgraphis 

 new to the British Isles, but known in N. America. A new 

 species of Ghssograplns (to be described in the forthcoming 

 part of the Palaeontographical Society's Monograph on British 

 Graptolites) was also discovered. 



The overlying grits }ield few fossils, shale bands being rare. 

 Diplographis dcniatns and Didyvwgraptns cxtensns, however, 

 were found. Further to the west, the shale partings increase 

 considerably, as seen on Rossroe peninsula. The same brec- 

 ciated limestone is found here, and the shaly partings in the 

 grits have yielded a considerable number of graptolites, 

 amongst others Diplograptus daitatits, Phyllograpius, Didyvio- 

 graptns extensns^ and D. fascicnlatiis. 



The Leenane and Rossroe Grits we therefore regard as of 

 similar age, both belonging to the Upper Arenig. 



The Upper Arenig conglomerates enclose pebbles of chert, 

 granite, quartz-felsite and quartzose schist, the last named 

 rock being identical with some of the quartzose schists of 

 Connemara. Dr. Callawa}-^ has noted the resemblance of the 

 granite and quartz-felsite pebbles to the Galway granite and 

 associated felsites, which are intruded into the crystalline 

 schists. Hence we may conclude that the schists of Conne- 

 mara were alread}^ metamorphosed before Arenig times. 

 General considerations lead to the conclusion that they are 

 actuall}" of Archaean age. 



^ Quart. Joiini. Gcol. Soc, vol. xHii., pp. 5 [7-524. 1.S87, 



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