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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 18, 



vertical, and the direction of which coincides, in a general way, with 

 the geographical axis formed by the two great valleys of Gweebarra 

 and Glenveagh just described. In addition to this stratified or 

 cleaved structure, it is traversed by many joints, which run nearly 

 at right angles to the directions of the planes of the cleavage-struc- 

 ture. The following Tables, which are constructed principally from 

 data collected in the Gweebarra district, show the directions both of 

 the cleavage- and the joint-planes. 



Table I. — Azimuth of Cleavage- and Joint-planes in the Granite of 

 Donegal. {Magnetic hearivigs.) 



Locality. 



Cleavage-planes. 



Joint-planes. 



1. Between Glenties and 1 



Gweebarra Eiver. . . J 



2. Gweebarra Valley South 



3. Gweebarra Valley, \ 



Doocharry Bridge J 



4. Sheskina-roan : Veins 1 



of Beryl and Quartz J 



, , Sheskina-roan 



10° South of West. 

 Dip 



Dip 



[ 30" S. of W, 



[^ vertical. 



15° S. of W, 



80° N. . 



5. Meen Bannad 



6. Anagarry Hill 



7. Lough Anure 



8. BetweenDunglowand I 



Doocharry Bridge J 



9. Between Doocharry! 



Bridge and Fin- V 

 town ". J 



10. Glenlehen 



11. FintownGap: Gneiss 



E.W. Vertical 



10°KofW. 

 plane . . 



10° S. of W. 



Fine' 



10° West of North. 



10°W. ofN. 



10°W. ofN. Dip 80° E 



10° E. of N. 



10°E. ofN. Dip 80° E. 



N.S. 

 15°E. ofN. 



7° E. of N. Vertical. 

 10°W. ofN. 



5"W. ofN. 

 10° W. ofN. 

 45°E.ofN. (Secondary.) 



10° W. of N. Vertical. 



5° W. of N. 

 15° W. of N. 



From the preceding Table, it appears that of the 15 joint-planes 

 observed, 9 lay to the west of north, 1 was due north and south, and 

 5 lay to the east of north : they may be thus arranged : — 



Table II. — Joint-planes in the Granite of Western Donegal, 



West of North (B). 



East of North (B). 



Secondary (A'). 



10° 



10° 



45° E. of N. 



10 



10 





10 



15 





10 



7 





5 







... 



10 







10 







6 







15 





... 



9°28'W.ofN. 



8°24'E. ofN. 



45° E. of N. 



Mean =3° 



4' W. of N. 





