144 



Froceedings of tlie Royal Irish Acadeiny. 



One of the most interesting sections in the great metumorpliic 

 series is to be seen on the south-west side of Croaghaun Hill, near 

 Achill Head. The promontory, known as the Head, is formed of gray, 

 silvery, knotted mica-schist, similar to that already described as form- 

 ing the Mullet, south of Binghamstown. This is followed in upward, 

 direct, stratigraphical succession by one or two limestone bands 

 which follow zig-zag courses, to be seen in the hollow striking 

 north-westward from Keem to the steep-sided inlet known as 

 Ooghnagerillen. 



Over the limestone and gray, silvery mica-schist associated there- 

 with are black, gray, and dark-greenish micaceous and chloritic 

 schists and a thick bed of coarse, pebbly grit. Following this thin 

 group is a fine conglomerate with rounded pebbles of quartz, felspar 

 pegmatite, and quartzite, introducing a great series of alternating 

 grits and conglomerates which forms the hill-side near the cliff-edge, 

 between the 1000 and 2000 feet levels, and cannot be much less than 

 1500 feet in thickness. The lowest grit beds of this series contain 

 amethystine quartz, which is much sought after locally. jN'otwith- 

 standing the great thickness of the conglomerate-grit series, the 

 conglomerate thins out eastward to such an extent that at 2^ miles 

 distant it is represented by a single thin band of boulder- 

 deposit, containing large, round blocks of light-gray, vitreous quartzite 

 embedded in a matrix of very similar quartzite. The blocks range 

 in size up to two feet by one foot three inches in sectional dimensions ; 

 and the deposit, which runs parallel to the road to Keem, and some 

 100 to 150 yards to the north of the road, is sufficiently peculiar to 

 warrant this notice. If it were not on the strike of, and in proximity 

 to, the great conglomerate series of the hill- side, it might be taken for 

 a broken-up condition of the local quartzite, resulting from intense 

 crushing, as in the case of the Howth quartzites, noticed by Professor 

 Sollas. Three miles east of the clifP-edge, on the west side of Dooagh 

 Bay, near its entrance, conglomerate occurs at the base of 

 the quartzite group, associated with cloritic and magnitite-bearing 

 mica-schist, which also contains irregular masses of conglomerate ; 

 and around these latter the schist moulds itself, as the result 

 of extreme pressure. These rocks correspond with those seen at 

 Keem Bay, below the great grit-conglomerate series of Croaghaun Hill. 

 The rapidity with which the series just mentioned thins out east- 

 ward is suggestive of the existence of ancient land to the west of 

 Achill ; but we cannot lose sight of the fact that prior to the deposi- 

 tion of the conglomerates was laid down a considerable thickness of 



