218 Proceedings of the Royal Irish Academy. 



granite proves to be almost a " granitised" amphibolite. Deep green 

 hornblende, of the type discussed in connexion with Cleengort, occurs 

 in it in rough foliation -layers ; but this mineral forms so much of the 

 roek as to carry the specific gravity up to 2'99 — the mean of three 

 closely agreeing determinations. Andesine is present, and, like the 

 hornblende, has probably recrystallized from an epidiorite, which 

 renewed its youth in the granite magma; while quartz and orthoclase, 

 representing the aplitic granite, are more abundant in the joint mass 

 than its specific gravity would suggest (PI. iii., fig. 2). A chain 

 of observations, and especially those made in the field, teach us to 

 regard this rock as composite. It is, then, one of the most extreme 

 modifications met with in the intrusive granite of Boylagh, 



It is interesting to observe how hornblende crystals, already formed 

 in andesitic lavas as products of the " first consolidation," become 

 frequently reabsorbed by the magma when it ascends or is poured out 

 upon the surface. They then leave, as von Lasaulx long ago pointed 

 out, a mere skeleton formed of grains of magnetite. But, in our 

 instances of the absorption of amphibolites by a granite magma, the 

 final crystallization takes place under considerable pressures. The 

 conditions are favourable for the crystallization of biotite, or even for 

 the reproduction of the hornblende, and a quartz -diorite with some 

 orthoclase results. Were this mixed rock again melted and thrust out 

 on the surface, a rhyolitic andosite with pyroxene would probably be 

 formed, in wliich a few corroded hornblendes from the previous con- 

 solidation might remain. 



Pegmatite veins and masses, belonging to a later granite, and 

 probably of Devonian age, are common throughout Derkbeg and Derry- 

 loaghan ; but undoubtedly the most striking features are those con- 

 nected with the foliation of the older granite. The constancy of strike 

 in this foliation becomes, if the foregoing observations are correct, a 

 record of the trend of the folded Dalradian series, which has here left 

 mere traces in the heart of the granite which attacked it. 



y. — The Gneissic Steuctuees neae Finntown, and theie beaeing on 

 THE Geological Histoey of Gneissic Geanite theotjghotjt 

 THE County op Donegal. 



In the eastern part of Galwolie, on the road from the village of 

 Lettermacaward to Doocharry Bridge, a red gneiss, containing musco- 

 vite as its predominant mica, reminds one of the crushed and slicken- 

 sided granite of Bamesbeg in northern Donegal. It similarly owes 



