TRANSACTIONS OF THE SECTIONS. 97 



On cei-tain pre-Carhoiiiferoas and Metamorphosed Trap-dyhes and the Asso- 

 ciated Hocks of North Mayo, Inland. By William A. Traill, M.A.I., 

 F.E.6.S.I., H.M. Geological Survey of Ireland. 



The author first described the locality in North Mayo lying between Downpatrick 

 Head and Broad Ilaveu, and referred to the geological map of Sir Richard 

 GrilBth. 



The physical features presented precipitous coast sections at Keady Point 352 

 feet high, and at I^euwee Head 829 feet high, in bold and perpendicular headlands. 



The geological t'ormatious comprising the older or metamorphic rocks lying to the 

 ■westward consist of flaggy quartzites and micaceous schists, often much contorted 

 and overlapped. The newer or Carboniferous rocks extending from the Glenglassera 

 river eiistward, with a primary dip E.N.E. at low angles, comprise white, yellow, 

 and red sandstones, vrith green and red shales, and limestone b.mds and beds 

 further eastward. 



These Lower Carboniferous sandstones rest uncouformably on the metamorphic 

 rocks, which is best seen along one side of a fault at Fohernadeevaun, at the mouth 

 of the Glenglassera river. The basal bed of the Carboniferous strata is a con- 

 glomerate of from 1 to 4 feet in thickness, merging into the overlying sand- 

 stone beds. 



With all due deference to the author of the geological map referred to, the presence 

 of the baud of 1 )evonian rocks, as there represented, intermediate between the meta- 

 morphic and Carboniferous rocks, was called in question and regarded as not 

 existing in that locality. 



The inlrusive igneous rocks of the district, though much resembling each other, 

 and both belonging to the basaltic type, were shown to belong to two distinct 

 epochs with regard to their time of formation, and also to possess characteristic 

 distinctions. 



The older set are undoubtedly of pre-Carboniferous age, as unmistakable frag- 

 ments are found in the basal conglomerate of the Carboniferous rocks, and they 

 seem to have existed before and to have been metamorphosed with those beds 

 among which thej' had been intruded. They largely penetrate the metamorphic 

 rocks, but are not found witliiu the area of the Carboniferous strata. They occur 

 chiefly in sheet-like dykes up to 150 feet in thickness, and are often contorted with 

 the beds they penetrate, though apparently the cause of some of the minor crump- 

 lings. They are seldom hexagonal, spheroidal, or amygdaloidal in structure. 



As examples of these, specimens were exJiibited and detailed descriptions given 

 of dykes on I5eniuore and Gleucalry, at 13elderg Harbom-, and Laghtmurragha, 

 showing the effects of the metamorphic action iipon them, viz. the change from a 

 hard splintery iidcrocrystalline basalt towards the centre, to a fibrous hornblendic 

 schistose rock, vv'ith soft green chlorite and nests of green chloritic mica, the ex- 

 terior becoming very schistose, platy, and micaceous, resembling a mica-schist ; in 

 part, theso might be considered diabases. The felspar is plagioclastic and has a cha- 

 racteristic white weathering in crystalline mottliugs through the greenish base. 



The adventitious minerals frequently present are mica, chlorite, epidote, garnets, 

 hornblende, quartz, calcite, varieties of felspar, and iron pyrites. 



The newer set, or post-Carboniferous dykes, are probably of Tertiary age, and 

 seem to fill cracks or tissiu-es or lines of faults. They are basalts mostly in vertical 

 dykes, and always cut the sheet-like dykes, and are frequently hexagonal, sphe- 

 roidal, or amygdaloidal in structure, and usually bear W.N.VV. and E.S.E., and 

 seldom exceed 25 feet in width. By their decomposing they separate many of 

 the small islands from their respective headlands by narrow precipitous gullies, or 

 form chasms into the cliifs : one of these clefts has vertical walls over 450 feet 

 high, and in part does not exceed some 10 feet in width; this same fissure also 

 cuts off some four islands from their adjacent headlands, the view down which is 

 almost unique. 



On the Sub-Wealden Rrploration. By H. Wtllktt, F.G.S. 



1876. 10 



