228 C. CALLAWAY ON THE GRANITIC AND 



and this weak point may have permitted the intrusion of the granite. 

 The schists of this locality have usually an easterly dip, and are 

 frequently intensely contorted. 



That this granite is intrusive is perfectly certain, and there is no 

 doubt that it is continuous with the promontory of gneissic granite. 



Granite of Dunaff Head and Malin Head. 



South-east of Dunaff Head rock is well exposed on the flat bare 

 shore. For some miles to the S.E. the region is occupied by quart- 

 zite ; but north of Lenan Head granite comes in, appearing amidst 

 the quartzite in irregular masses and sending out veins. On the 

 same strike, to the N.E., granite is exposed on the south side of 

 Malin Head, and is clearly intrusive in the quartzite of the district. 



Prof. Bonney has examined microscopic sections of granite from 

 all the localities described, except the last, and he is of opinion that 

 they " exhibit traces of an original igneous structure, which appears 

 to have been much modified at a later time." 



4. The Foliation in the Geanite. 



It seems to have been generally assumed that the presence of a 

 gneissic structure in a rock is a certain proof of its metamorphic 

 origin ; but this opinion has never been justified by facts. There is 

 no a priori reason why an intrusive igneous rock should not be foliated. 

 Of late years we have acquired larger views of the eff'ects produced 

 by pressure. A lateral force which could thrust regional masses of 

 Hebridean gneiss over newer rocks to the breadth of a mile, and 

 squeeze a north-western into a north-eastern strike along a line of 

 many miles in length, may well be capable of producing a foliated 

 structure in a band of granite some three or four miles in width. 

 Certain parts of the Donegal granite mass are incontestably in- 

 trusive in the schists. Which view, then, must we adopt : that the 

 granite is igneous, with a gneissic structure subsequently superin- 

 duced ; or that it was originally part of a mass of bedded sediments, 

 some portions of which have been heated to the point of fusion ? 

 As the question is one of considerable theoretical interest, a careful 

 review of the evidence is desirable. 



Arguments * for the Metamorphic Origin of the Granite. 



(1) The Granite is " gneissose in character." 



This fact I hold to be consistent with the igneous theory, 



(2) It occurs " in thin beds corresponding to the bedding of the 

 stratified rocks of the district." 



If this means that the strike of the foliated granite corresponds 

 with the strike of the district, it is sufficient to say that this would 

 also be the case on my hypothesis. But if these words imply that 

 the granite is truly stratified, I reply that, in the typical section at 

 Barnesbeg Gap, the granite lies between the masses of schist, not in 

 beds, but with the irregularity of an intrusive rock. 



(3) In the granite oligoclase occurs associated with quartz. This 



* R. H, Scott, Journ, Geol. Soc Dubl. vol. ix. 



