222 Proceedings of the Royal Irish Academy. 



altering to actinolite,^ quartz, and a lime-scapolite, have arisen in it, 

 together with sphene, which Mr. Scott* always associated with altered 

 limestone in Donegal. Labradorite felspar occurs abundantly in parts 

 of this rock, side by side with quartz, but seems to vary in amount 

 in inverse proportion to the scapolite. 



This is probably the locality where Mr. Scott^ discovered scapolite 

 in 1861, in conjunction with sphene, pyroxene, orthoclase, and quartz. 

 After passing Carbal Gap, across the Glenleheen Eiver, a fine section 

 is seen, partly quarried, on the north-east side of the road, in the 

 townland of Loughnambraddan. The structural planes are vertical, 

 and a remarkable variety of rocks can be gathered within a few feet 

 of one another in traversing the strike. After a little scrutiny, two 

 types of rock become sorted out, the one a pink euritic aplite, with 

 the characteristic specific gravity of 2"59, the other a dark 

 hornblende-biotite-schist, with a specific gravity of 2'89. 



The former shows under the microscope the structure of a mildly 

 fluidal gneiss, without banding, but in which a few of the felspars 

 assumed ovoid contours, from continued movement after crystallisation 

 had commenced (PL v., fig. 1). The quartz settled down in angular 

 interlocking grains, like those of a metamorphic quartzite. Only the 

 merest trace of biotite occurs, and the aplitic character is complete. 

 Iron ores are represented by rounded grains of pyrite. 



The dark rock, on the other hand, is an obvious schist, 

 almost slaty in places, though more distinctly crystalline than 

 some of the masses on Carbane (p. 212). Under the microscope it 

 shows a predominance of hornblende over biotite ; these minerals are 

 associated with abundant epidote, sphene, and pyrite. The three 

 last-named constituents seem alike to have existed in the rock before 

 its invasion by the granite. The usual basic felspars, and a few 

 granules of interstitial quartz, form a second association of minerals, 

 interwoven, as it were, with the ferriferous ones. So far, the rock 

 is a typical schistose epidiorite (PI. v., fig. 2). 



But other bands in this striking roadside section show a coarse- 

 grained granite with pink orthoclase, intruding up the general vertical 

 planes. In hand-specimens this rock resembles some of the handsome 

 gneisses of the Outer Hebrides ; but its most foliated portions are 



1 The distinct green colour of this paramorphic product indicates the presence of 

 iron also in the diopside. 



* Op. ciL, Journ. Geol. Soc. Dublin, vol. ix., pp. 288 and 289. 

 ^ Op. cit., ibid., vol. x., p. 21. 



