Yol. 65.] ROCKS OF THE TOTJRMAKEADY DISTRICT. 12f 



III. The Field-Relations of the Crystalline 

 Igneous Rocks. 



The crystalline rocks include : 



(«) a great series of felsites, many certainly intrusive, others 

 contemporaneous ; 



(b) a small development of intrusive andesitic rocks ; 



(c) a number of small but interesting intrusions of hornblende- 



lamprophyre, and dolerite. 



(a) The Felsites. 



The rocks which play the most prominent part in the Tourma- 

 keady district are undoubtedly the felsites. They form a number 

 of relatively small intrusions along the western and south-western 

 borders of the district ; but it is in the central part, from near 

 Tourraakeady Lodge to Shangort, that this type of rock is especially 

 prominent, forming nearly all the high ground, and extending 

 with lessening importance as far as the neighbourhood of Derrin- 

 daffderg. Lithologically, the rocks belong to three fairly well- 

 marked types : — (a) the green and brown felsite, (/3) the red felsite, 

 and (7) the augite-felsite. 



The field-relations of these rocks may now be briefly described. 



(a) The green and brown felsite. — This rock-type is 

 characterized partly by the prevailing colour of the ground-mass, 

 but chiefly by the size and prominence of the quartz-phenocrysts. 

 Though it occurs here and there all over the district, its principal 

 development is north-west of a line drawn between Shangort and 

 Gortbunacullin. Here it forms a great oval mass, having a length 

 of about a mile and a maximum width of nearly half a mile, 

 extending from the neighbourhood of Derrassa to Stream G, north- 

 west of Gortbunacullin. South of the stream the mass tapers 

 and becomes more irregular, but extends as far as about half a mile 

 south-west of Gortbunacullin, and therefore the total length of 

 the mass is about a mile and a third. All along its northern 

 and eastern border it truncates the ends of the bedded rocks, tuffs, 

 grits, and cherts, but along its western border its relations to the 

 coarse (?) Bala conglomerate are nowhere visible, owing to the 

 covering of peat, except at certain points in the upper parts of 

 Streams G and H. In the latter stream, however, the felsite shows 

 what appears to be a chilled edge, suggesting that it is an intrusion 

 of later date than the (?) Bala conglomerate. The section is not, 

 however, so clear as could be wished, and can hardly be regarded 

 as conclusive on this point. 



North of Gortbunacullin patches of chert are seen caught up by 

 the green felsite, exposed in the bed of Stream G. 



