CoLK — On Composite Gneisses in Boylagh^ West Donogat. 207 



<'ffect has produced a vei*y fine-grained biotite-gneisB along the junction. 

 There is in the field no question whatever of basic segregations in the 

 granite, such as are so often relied on to explain variations in an 

 intrusive mass. The phenomena are those of intrusion along the 

 pre-existing foliation-planes of a schist ; and a good giieissose rock 

 results, set with iiiclusion-flcclcs, in which these foliation- planes are 

 still apparent. Subsequent pressure may have intensified the eye- 

 structures, and may have broken up some of the residual and included 

 layers of schist ; but this gneissic border to the granite is none the 

 less of composite origin, and its main characters remain due to the 

 circumstances of its original flow.' 



On the north side of the Ardara dome, towards Gweebarra Bay, 

 tlie same phenomena are repeated. Uncontaminated granite, like that 

 of Carn in southern Donegal, is seen in the veins that come up 

 through the Dalradian series at Portnoo. The specific gravity of 

 tliis beautiful white rock is 2-59. It consists almost entirely of 

 quartz, orthoclase, and microcline, with a mere trace of greenish 

 biotite. Muscovite has developed, however, as an alteration-product 

 ill the felspars. Another type at Cashelgolan, east of Clooney, is a 

 pale granite with well-developed primary muscovite ; here the rock 

 intrudes into a schistose series. 



The granite at Portnoo, on the shore on the east side of Dunmore 

 Head, cuts in numerous veins and dykes across a tough diorite, and 

 similarly invades the gi'ay crystalline limestone of the headland. In 

 the former case little absorption takes place, except on the margins of 

 some of the dykes, and the rock remains undarkened ; in the latter 

 case the granite has detached small fragments of the limestone, and 

 has even entered along the fissures where the successive beds in a 

 small anticlinal have "sprung" apart. But no appearance of inter- 

 lamination on a large scale has arisen, and the gxanite remains devoid 

 of foliation. Gray garnet and wollastonite, the latter in microscopic 

 prisms, have arisen in the limestone, some layers of which have 

 become so rich in silicates as to be practically flinty. The vai'iation 

 in the type of alteration in alternate layers is, however, due to original 

 differences in the composition of the strata, and not, as might be 

 supposed in the field, to parallel intrusions of the granitic magma. 



1 Compare the observations of Lacroix on the granite with elongated inclusions 

 in the valley of Boutadiol, etc., " Le granite des Pyrenees et ses phenomenes de 

 contact," 2"« memoire (1900), Bull. Carte geol. de la France, No. 71, pp. 21 

 and 25. 



