SECTIONAL TK ANS ACTIONS.— C. 543 



Mr. R. M. Craig. — On the Occurrence of Flinty Crush-rock in the Outer 

 Hebrides. 



The Outer Hebrides, so far as thej^ have been examined, appear to consist 

 mainly of a complex of gneissic rocks similar in most respects to those already 

 described from the western seaboard of Sutherland and Ross. They are mainly 

 orthogneisses of various types, but paragneisses also occur, including crystalhne lime- 

 stones, quartz schists and graphite schists. Later are dykes of quartz dolerite, 

 tholeiite, olivine dolerite (often of Crinan type) and camptonite, most of which are 

 probably of Tertiary age. 



Probably the chief feature of interest in these islands is the great zone of shearing, 

 due to earth movement from E.-W. or S.E.-N.W., which traverses the E. coast in a 

 generally N.-S. direction. The shearing is accompanied by extensive crushing of 

 the rocks of the gneissic complex leading to the production of mylonites on an extensive 

 scale. A further stage in dynamic action is shown in the production from the finely 

 ground material of flinty crush-rocks. In places these show signs of fusion, forming 

 pseudo-tachylyte, which behaves intrusively to the surrounding rocks. Exceptionally 

 the fused material (pseudo-tachylyte) exhibits signs of recrystallisation in the presence 

 of spherulites and in groups of microlites of felspar and hornblende. 



Mr. Gerald Andrew. — (a) Note on a Basic Sill in North-west Donegal, 

 (b) The Contact Relations of the Donegal Granite. 



(a) 



A member of the early basic sill intrusions into rocks of Dalradian type in N.W. 

 Donegal is exceptionally fresh. The rock is an ophitic quartz-dolerite, with faintly 

 brownish augite, oligoclase-labradorite, and interstitial micrographic quartz-felspar 

 intergrowths ; accessories are opaque ore (ilmenite), apatite, biotite and hornblende. 

 The last two occur apparently independently, as well as in association with other 

 minerals, the biotite fringing and partly intergrown with ilmenite, and the hornblende 

 occasionally fringing the augite. The hornblende is pleochroic, brownish green to 

 green, and is not uralitic in character or origin. 



The locality of the best example is on the south-east shore of Marble Hill Strand 

 (1 in. Sheet 4), S.E. of Dunfanaghy. Similar dolerite occurs on the western base of 

 Errigal (1 in. Sheet 9), N. of Dunlewy. On the promontory of Rosguill (Sheet 4) sills 

 have fresh augite cores, the felspar being decomposed, but the original structure of 

 the rook is retained. 



The sills are cut by acid minor intrusions (connected with the granite) in Rosguill 

 and at Marble Hill Strand, and are members of the group which has elsewhere suffered 

 shearing with the schists. They are therefore earlier than the main movement, 

 which was pre-Caledonian (c/. Fearnsides and others, Proc. R.I. A. 26, p. 100). 



The lowest grade members of this group known hitherto occur on the W. Coast 

 of Scotland (Mem. Surv. Scot. Sh. 36, pp. 50, 52 ; Sh. 37, p. 65, &c.). 



(b) 

 I. Characteks of the N.W. Part of the Main Granite Area. 



The granite is a pink granodiorite, in parts strongly foliated parallel with the 

 margin, and with the foliation of the country rock. Large xenoliths are common, 

 with their foliation parallel with that of the granite in most cases, but at an angle 

 with it in some (Mem. Oeol. Surv. Ireland, Sheets 3, 4, 5, &c., p. 71). The country 

 is intruded lit-par-lit at the margin. XenoUths of biotite-schist, one of the commonest 

 rocks outside the granite, are rare, unless certain biotite-rich grey gneissose rocks, 

 previously described {Me7n. cit., p. 134) as crushed granite, are such xenoliths. 

 Xenoliths of quartzite, limestone, flagstone and amphibolite show no signs of assimila- 

 tion, except in the case of the pegmatites intersecting limestones, and certain amphi- 

 bolite xenoliths, the field relations of which are obscure. 



II. Characters of the Rosguill Granite Area. 



The foliation of this mass is weak, xenoliths are exceptionally common, and 

 swarms of small (2-6 in.) xenoliths occur (polygenetic agmatite). There is no 

 marginal foliated biotite-gneiss, the country being mainly pelitic hornfels poor in 

 biotite (andalusite or sillimanite hornfels). In the case of at least one amphibolite 



