248 



GRANITE VEINS IN SCOTLAND. 



Mayo. A considerable mass of granite extends from N.E. to S.W. 

 in County Mayo. Professor Hull describes 1 that of Aillemore as 

 forming two mountains named Coroock Brack and Knockaskeheen. 

 It is of a grey colour, usually fine-grained, consists of quartz, ortho- 

 clase, and probably oligoclase, and dark-green mica. Occasionally it 

 has the aspect of graphic granite. It is traversed by joints which run 

 IS", by 10 W. It is surrounded by schists, and is older than the May 

 Hill sandstone. 



The orthoclase under the microscope is often cloudy, but some- 

 times shows cross-banded structure, as in the Firbogh granite. The 

 quartz is similar to that of Galway. The mica often includes grains 

 of magnetite. 



This rock is said by Mr. Symes to be a true eruptive granite, 

 without a trace of foliation, and sends veins into the surrounding 

 rocks. 



Granite Veins. 



Granite Veins. It is difficult to find a satisfactory example of 

 any extensive tract of granite without the occurrence of granite veins 

 ramifying through the neighbouring rocks. They occur in Cornwall, 

 Cumberland, and Arran, in Ben Cruachan, at Strontian, in Glen Tilt, 

 and generally throughout the Highlands. The same is true for the 

 continent of Europe ; and perhaps we nowhere find a better example 

 of the elevation of granite in a solid form, than that described by 

 Murchison at the Ord of Caithness. This granite, on its northern 

 flank, supports the old red conglomerate, whilst to the south it 

 occupies a cliff on and near the shore, the verge of which affords 

 a remarkable breccia, compounded from all the beds of the oolitic 

 series that occur on this coast. This breccia of sandstone, shale, 

 and limestone, is tilted off from the granite wherever that rock 

 protrudes upon the shore, whilst the strata are regularly developed 



where the granite recedes into the 

 interior. No veins or portions of 

 the granite are to be met with 

 in or above the oolitic breccia, 

 which, by its disturbed position, 

 appears to fix the maximum of 

 antiquity of the elevation of the 

 granite as not older than the coral- 

 line oolite. 



Tornidneon. The granite veins 

 of Tornidneon in Arran pass from 

 a body of very coarse - grained 

 granite through nearly vertical 

 laminae of dark quartzose clay 

 K & slate \ the line of junction dividing 



the whole side of a hill. One 



he veins encloses fragments of slate, and divides itself into 

 1 Hull, Journal of the Geological Society of Ireland, vol. iv. p. 4. 



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