SECTIONAL TRANSACTIONS.— C. 545 



agglomerates. Non-porphyritic varieties are locally slaggy and are considered to 

 be Tertiary plateau lavas let down within the vents and so preserved ; (2) finely 

 brecciated country rock bordering the ring-fissure to north and east, considered due 

 in part to explosion, in part to fault-movement along the ring-fissure ; (3) porphyritic 

 hornblende-granophyre ring-dyke along three-quarters of the ring-fissure in contact 

 with (2), brecciated next to (4) ; (4) agglomerates and explosion breccias in vents 

 along S.W. part of ring-fissure (Forkhill district), chiefly of fragments of coimtry rocks, 

 but including also trachytic rocks that presumably originated from the vents ; 

 (5) porphyritic felsite ring-dj-ke, accompanying and intruding (4). 



The earlier ring-dyke (3) where best seen in section (Cam Lough) is inclined north- 

 wards, i.e. outwards, at about 70 degrees. There is no evidence of any marked 

 subsidence of the area enclosed by the ring-complex. The ring-dykes follow a ring- 

 fissure that has been enlarged, locally at any rate, by volcanic action. 



The N.W. belt consists of intrusions of biotite-gabbro (Slieve Gullion^ quartz- 

 gabbro (Foughill ; E. of Flurrybridge), and non-porphyritic or sparsely porphyritic 

 granophyres (SUeve Gulhon, Foughill, Anglesey Mountain). Structural and time 

 relations have not j'^et been fully worked out. Granophyres, however, are well seen 

 to N.W. and S.E. in intrusive contact with the porphyritic granophyre ring-dyke, 

 and are clearly later in age. 



It would seem that the ring-complex marks the first intrusive phase, and that 

 subsequently igneous activity became localised along a N.W. line of weakness. 



Mr. T. M. FiNLAY. — Rolled Spherulites in Felsite from the Shetlands. 



These abnormal structures occur in a series of dyke rocks cutting both the Ronas 

 Hill granite mass of the north-west of Shetland and the gneiss into which the granite 

 is intruded. The first specimens noted occurred as scree material, and the features 

 displayed on the weathered surfaces suggested the preservation of an organicallj-- 

 forraed rock by hornfelsing, so strongly did they simulate organic structures. Later, 

 however, a series of dykes, varying in width from 50 ft. down to 18 in., was found 

 in situ, placing the intrusive, igneous origin of the rock beyond doubt. 



The rock is generally blue in colour, is extremely hard, and often splits readily 

 into slabs of varying thickness. Incidentally these ciiialities of hardness and fissilit}'- 

 made the rock a suitable one for implement-making ; polished flensing knives made 

 from it are peculiar to the districts of Northmaven and North Roe, where the dj-kes 

 occur. 



Microscopic examination of the normal type of rock in all the dykes shows that 

 the dominant structure developed is the spherulite. These structures are composed 

 of delicate radiating needles of Riebeckite forming beautiful spherulites up to ^ in. 

 in diameter, embedded in what is now partly quartz, partly a micropegmatitic inter- 

 growth of quartz and felspar on an exceedingly minute scale. Where this normal type 

 of consolidation is departed from abnormal structures abound. These take the 

 form of (rt) broken spherulites, the riebeckite needles being spread out along lines of 

 flow ; (6) rolled spherulites, where a spherulitic or glassy spheroidal nucleus has been 

 rolled over and over in a layer of viscous magma giving rise to spiral structures as 

 seen on a fractured surface ; (c) enlarged spherulites where a similar nucleus is bordered 

 by radiating tubules filled with quartz and micropegmatite (' soUd vesicles '), the 

 whole bounded by a distinct glassy wall ; and finally (rf) areas, laminar, finger-like 

 or irregular in outline with a remarkable pseudo-cellular internal structjre residting 

 from the arrangement of the riebeckite microlites ; the outer surfaces of these patches 

 are also studded with a system of tubules (' solid vesicles ') often septate and bent in 

 the direction of flow. 



Analysis shows the rock to be highly acid (SiO^ 76%), rich in alkalies, magne-sia 

 absent, and lime nearly so. Wliile no attempt is made at present to arrive at the 

 origin of these structures, it is suggested that they are paralleled by the hthophysal 

 and spherulitic structures so often noted in rhyoUtes, and that the same factors are 

 operative in their formation. 



A similar rock is known from Northern Nigeria, also a dyke rock and associated 

 with granite ; in the specimen examined only the spherulitic type of crystallisation 

 was developed. 



Afternoon. 

 Excursion to Bishopbriggs and Torrance. (Leader : Prof. J. W. 

 Gregory, F.R.S.) 



1928 N N 



