544 SECTIONAL TRANSACTIONS.— C. 



xenolith, which is particularly clearly exposed, local assimilation has taken place. 

 The centre of the xenolith is pierced by granitic veins 1-3 in. thick. These become 

 closer near the margin, and the critical stage is that of an agmatite, in which biotite 

 is produced at the expense of the hornblende of the amphibolite, and the orthoclase 

 of the granite, which is absent from the veins (c/. Harker, 'Tert. Ig. Rocks Skye,' p. 171). 

 The felspar of the xenolith is a strongly zoned (Andesine-Ohgoclase) plagioclase. The 

 clots appear to be disseminated through the granite as biotite-rich patches smaller 

 in size the further into the granite they occur. The granite is white where xenolithic, 

 but is otherwise pinkish. The zone of mixing is approximately 4—6 yards along the 

 strike of the xenolith, and less transversely. 



III. Conclusions. 



The granite was intruded later than the tectonic maximum. The foliation of the 

 biotite-rich rocks is not attributable to crushing of the granite, and is considered to 

 be a relict structure. The rocks themselves are either simple or injected xenoliths of 

 biotite -schist. They occur where xenoliths of schist are to be expected, from the 

 nature of the country rock. Nearest the margin of the granite they occasionally carry 

 pink porphyroblasts of oligoclase. Farther within the granite a banded gneiss 

 (1-3 in. bands), with alternating grey biotite-rich bands and pink granitic bands, occurs. 

 In the grey portion small grains of pink felspar are common, and a slice showed a 

 small clot of spinel and corundum, with epidote. The grey portion is considered to 

 be the last recognisable stage of the biotite-schist in the processes of incorporation. 



It is considered that the evidence establishes that (1) under the low-grade condi- 

 tions obtaining in N.W. Donegal certain rocks are refractory, e.g. quartzite, Hmestone, 

 flag, aluminous shale, amphibolite ; (2) certain biotite-schists, of mineralogical 

 composition similar to that of the granite, tend to be assimilated, and other rocks 

 (e.g. amphibolite) follow suit if able to form a mineral aggregate similar to that of the 

 invading rock. The former process depends on mechanical (Ut-par-lit) incorporation 

 mainly, and on the use of the granite juices (water, &c.) for injection ; the latter on 

 chemical reaction with the fluid part of the magma. Lit-par-lit injection on a fine 

 Bcale is especially favoured by the existence of previous strong schistose or bedded 

 structure in micaceous (or chloritic) schists or slates. Under the contact effect of 

 the granite the mica tends to grow, but in one plane, and thus emphasises the fissility. 

 In other rocks (e.g. quartzite, &c.) a granular aggregate is produced by the production 

 of equidimensional crystals (homfels structure) in the main mass of the rock, with 

 consequent welding of the planes of fissility, and ' lit ' structure is coarse, or does not 

 occur. 



The phenomena are similar to those described by Cole in S.W. Donegal (Proc. R.I. A. 

 24 B, pp. 203-230; Proc. R.I.A. 25 B., pp. 117-123; Proc. R.I.A. 24 B, pp. 

 361-370), and the above account may be considered as supplementary to these. 



Mr. J. E. RiCHEY. — Ring-dyhes of Slieve Gullion, Ireland. 



The Slieve Gullion Tertiary Igneous Complex is intruded aroimd the south-west 

 end of the Devonian granite of Newry and mainly consists of : ( 1 ) a ring-complex 

 of vents and two acid ring-dykes, seven miles in diameter, that mark the course of a 

 ring-fissure ;i and (2) a north-westerly extending belt of basic and acid intrusions 

 bisecting the area enclosed by the ring-complex. Basic minor intrusions include 

 N.W. dykes and centrally-inclined sheets. The area was mapped and described by 

 Nolan, Egan and Traill during the primary survey of Ireland fifty years ago.^ The 

 writer's preliminary reinvestigation has been more especially concerned with the 

 age-relations of the larger rock-masses mapped, determined in the case of intrusions 

 mainly by chilled margins. 



The time-scale now established for the ring-complex is as follows : (1) Basalt 

 (Forkhill district), non-porphyritic olivine-rioh and non-porphyritic and porphyritic 

 olivine-poor, in small masses, associated with and yielding fragments to the vent- 



1 Recognised by W. B. Wright in ' Tertiary and Post-Tertiary Geology of Mull, &c.' 

 (Mem. Oeol. Surv.), 1924, p. 7. 



'^ Geological Survey of Ireland, one-inch map, Sheets 59, 60, 70 and 71, and 

 Explanatory Memoirs (1875-78). See also J. Nolan, Journ. Roy. Oeol. Soc. Ireland, 

 vol. iv, 1877, p. 233, and Oeol. Mag., Dec. ii, vol. v, 1878, p. 445 ; and Sir. A. Geikie, 

 ' Ancient Volcanoes,' vol. ii, 1897, p. 422. 



