366 Proceedings of the Royal Irish Academy. 



■wMcla is probably a relic of amphibole, absorbed into the aplitic intruder. 

 Again, on tbe rocky crest about one mile from Drumahair, beside tbe 

 mountain-road to Lurganboy, the various stages of absorption can be 

 traced with the unaided eye. Lumps of amphibolite seem to swim in 

 the gneiss, and to fade ofl into it, as if melting before our eyes. The 

 gneiss becomes enriched with streaks of basic matter, in which biotite 

 begins to predominate over amphibole. Over a wide area it passes 

 into a banded biotite gneiss, in which the lens easily reveals the pale 

 brown garnets, derived from the amphibolite, still surrounded in most 

 cases by a retinue of biotite-flakes. A mile westward, in Stonepark, 

 down against the road to Lough Gill, the evidence of the intrusive 

 character of tbe Leitrim gneiss is still more marked. The blocks of 

 dark included rock here consist of eclogite, containing a deep green 

 pyroxene, abundant garnet, magnetite, quartz, granular triclinic 

 felspar, and a variable but subordinate amount of hornblende and 

 brown mica. On the margins, in contact with the gneiss, biotite has 

 freely developed, so that some lumps, before they are broken across, 

 resemble mica-schist. Biotite similarly appears along the margins of 

 the aplitic veins sent off into the larger masses of eclogite from the 

 gneiss. 



The gneiss of Stonepark is in consequence beautifully flecked with 

 dark absorption-products, grouped along the lines and surfaces of flow. 

 Under the microscope these black flecks prove to consist largely of 

 biotite and garnet (fig. 3), as in the slide prepared by the Geological 

 Survey from the Slish wood mass. Muscovite, however, is also present, 

 and here and there a prism of pale pyroxene remains. Isolated 

 garnets lie in the gneiss, which is also speckled by a number of 

 crystals of a spinelloid. This black mineral, by its red alteration- 

 products, seems to be ordinaiy magnetite, which is an abundant con- 

 stituent of the amphibolites. 



I have similarly no hesitation in assigning a composite origin to a 

 rock styled " homblende-omphacite-gneiss," No. 1966 of the Survey 

 collection, from the east end of the metamorphic area of the Rosses. 

 Another slide in the same collection, from the south, of Lough Cooney, 

 and about one and a half miles south-west of Ballysadare, shows clearly 

 the derivation of garnetiferous material from the amphibolite. The 

 label, "amphibolite penetrated by granite," indicates that a revision 

 of the area by the officers of the Survey would probably have led to 

 the conclusions expressed in the present paper. No suggestion, how- 

 ever, as to the relations of the granites to the amphibolites is given in 

 the " Guide to the Collections of Rocks, and Fossils," published in 



