TIIK ORDOVICIAN SYSTKM 139 



among the Carboniferous rocks of Kildare. The succession there 

 is shown in Fig. 43, reproduced by permission of Messrs. Reynolds 

 and Gardiner 19 who have described the rocks seen. All the beds 

 belong to the Bala Series, and have a thickness of about 2200 

 feet, the main limestone being about 550 feet thick, and containing 

 many trilobites and brachiopods ; the fauna is that of the Bala 

 limestone, nor do the overlying shales yield Ashgillian species, 

 though beds of a higher horizon may be present 



Northern Facies. In the counties Down, Armagh, Monaghan, 

 and Cavan the thickness of the Ordovician Beds is reduced to a 

 band of black shales and mudstones exactly resembling those of 

 Moffat and the southern Scottish belt. Many years ago, from 

 exposures south of Belfast Lough, Mr. Swanston obtained all the 

 characteristic graptolites of the Glenkiln and Hartfell shalrs. 

 There are also some narrow inlying anticlinal exposures among 

 the neighbouring Silurians, and in one of these near Slane (County 

 MMtli) Mr. M'Henry has found some layers of banded Radiolarian 

 chert and dark shales yielding Lower Llandilo or Arenig 

 graptolites. 20 



The main band of Ordovician runs from north-east to south- 

 west, and in Monaghan and Cavan is from 3 to 9 miles broad. The 

 strata pass north-westward below the Carboniferous rocks, and a 

 portion of them emerges on the southern side of the Archaean 

 ridge near Pomeroy in Tyrone. The Arenig and Llandilo Series 

 appear to have thinned out and to be overlapped by the Bala 

 deposits, which rest directly on the old rocks. The beds are about 

 350 feet thick and have a conglomerate at the base, overlain by 

 sandstones, flagstones, and mudstones which have yielded a fauna 

 comparable to that of the Drummuck Beds of Ayrshire (Ashgillian) ; 

 moreover they are conformably succeeded by Silurian strata. 21 



Western Facies. Ordovician rocks occupy a considerable 

 area on the borders of Mayo and Galway between Killary Harbour 

 and Lough Mask, and there is another tract between Clew Bay 

 and Castlebar. The most complete succession is found in the 

 cliffs and escarpments of the tracts along Killary Harbour. On 

 the south side the lowest beds seen are black shales and cherte 

 about 60 feet thick, containing four species of Tetragraptxs with 

 Dichograptus 8-brachiatus. These beds are succeeded by coarse 

 conglomeratic grits with bands of shale which have yielded 

 Didymograptus extensus and Diplograptus dentatus ; this group is 

 2500 feet thick, and the grits contain pebbles of red granite and 

 quartz-felsite like those which occur in the Archaean rocks to the 

 north and south. 22 



On the south side of the Harbour these grits seem to be 



