SCHISTOSE ROCKS OF NORTHERN DONEGAL. 



223 



Donegal. 'The revelation made by the rocks themselves was both a 

 surprise and a disappointment. I have visited Prof. Hull's most 

 critical sections, and some districts not described by him. 



Pig. 1. — Section in Bar 



Gap and on Lough Greenan, 



S.E. 



Lough G-reenan. Lough Salt. 



I 

 I 





G. Granite. 

 L. Limestone. 



Ms. Mica-scliist. 



Hs. Hornblende-schist. 



Section in Barnesheg Gap (fig. 1, north-west end). 



It was in this wild ravine, near a wooden barrack erected for a 

 body of soldiers appointed to protect a cottage opposite, that I fi.rst 

 touched the granite. The rock surfaces on each side were well 

 rounded by ice-action, and the colouring and weathering suggested 

 some bits of Hebridean scenery in Scotland. The first blow of the 

 hammer destroyed my preconceptions. The rock was grey in colour, 

 uniform in texture, consisting of a well-crystallized, coarse-grained 

 aggregate of quartz, black mica, and felspar, mostly orthoclase. 

 There were no signs of bedding, and, but for a roughly linear 

 arrangement of the mica, I should have declared I had before me as 

 typical a granite as I had ever seen. 



Towards the junction with the schists at the south-eastern end 

 of the Gap, the granite grows lighter in colour and less coarse in 

 grain. At the entrance of the gorge we reach the important part 

 of the section. Masses of schist lie between masses of granite, and 

 at a hasty glance the two appear to be interstratified. I first came 

 upon a thin band of well-crystallized mica-schist, dipping at a 

 moderate angle to the south-east, with granite on both sides of it. 

 A little further on, I reached hornblende-schist, the hornblende 

 (consisting of small black sparkling crystals) predominating over the 

 quartz. This rock at first was rather puzzling. It had no regular 

 dip, but rose vertically like a dyke, with irregular sides, as if it 

 had thickened abruptly downwards. But this diorite-like mass was 

 distinctly foliated, and I at first thought that I had before me a 

 case of pressure-foliation in an igneous rock. The foliation, how- 

 ever, was not parallel to either of the margins of the mass or at 

 right angles to the direction of the thrust which had contorted 

 the region, but sloped at a medium angle to the south-east, in 

 accordance with the prevailing dip of the schists outside the granite. 



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