Cole — On Co)npoule GnekHefi in Boi/Iaijh, Wrd Donprjal. 211 



caused a further development of grcenisli biotite in a thin contact- 

 zone along their junctions with the schist; in fact, this biotite seems 

 to have been deposited as a first product of cooling from the veins 

 themselves; just as the augite in the dolcrite veins traversing oldej- 

 doleritc at Portrush has a tendency to gather on their margins. Then 

 the felspar, which is mostly orthoclase, formed a zone on. each side, 

 leaving the quartz, which has separated last, to occupy a central band. 

 Some large crystals of biotite occur, set irregularly in the veins. This 

 little block, measuring some 7 cm. by 7 cm. by 6 cm., shows us that 

 many lumps of schist must have been altogether cut to pieces and lost 

 during the invasion by the igneous mass. 



The dark microcline-granite in Ballyiriston shows under the micro- 

 scope the clustered groups and flecks of biotite, associated with epidote, 

 with which one soon becomes familiar along such intermingled contact- 

 zones (PL III., tig. 1). A little sphene and green hornblende have 

 developed, indicating the approach to more basic types. The specific 

 gravity of this granite is slightly raised, and is here 2-69, that of 

 the normal granite of Boylagh, as tested from various localities, being 

 close on 2-60. The pure microcline-granite is here, then, modified 

 towards quartz-diorite ; at the same time, many of the inclusions in 

 it become so commingled with matter from the granite as to pass 

 at their margins quite insensibly into the igneous mass. 



III. CAEBAlfE, NEAH GlENTUES. 



A concrete example of the structure of Boylagh occurs on Carbane, 

 a hill 450 feet in height above the sea, and half a mile east of the 

 little town of Glenties.^ The Dalradian series in " the Eock," as the 

 rough street at the south end of the town is styled, consists of vertical 

 well-bedded quartzites and black micaceous shales. Along their strike, 

 as they swing round to the north-east, they show crumplings, and the 

 shale-layers pass in a quarter of a mile into foliated and wi'inkled 

 mica-schist. The pressure under which this change was brought 

 about is evidenced by the folding of the metamorphosed strata round 

 small eyes of amphibolite (epidiorite) ; these doubtless represent the 

 characteristic accompanying sheets of dark igneous rock, diorites and 

 aphanites, which have become broken up in the more yielding shales. 

 " The Rock " at Glenties is probably itself an eye on a large scale, a 

 patch of strata that has escaped crumpling and deformation. 



1 This is the area referred to in the Survey Memoir to S.W. Donegal, pp. 29 



imd 53. 



