of the North of Ireland. 227 



times very considerable. That on the N.W. side of Arragh rises 

 perpendicularly 40 feet, like a partition wall ; that on the cupola of 

 Bally-ghuia 8 or 10 feet ; at Scrabo Hill near Newtown Ards in 

 the county of Down a dyke appears like a standing pillar at the 

 entrance of one of the freestone quarries. The dyke of Port-na- 

 brock near the Giant's Causeway, juts out into the sea quite iso- 

 lated to the visible extent of 372 feet. On the contrary, those on 

 the summits of Glendoun, Glencarn, and Aghla-mor, appear like 

 strewed masses, scattered about upon the surface. These might be 

 adduced as instances to prove the wearing away of mountains, if 

 that point stood in need of any additional confirmation. 



The depth to which the dykes descend is unknown ; and after 

 having observed the sections of a great many along the coast in 

 cliffs from 50 to 400 feet in height, I have not been able to ascer- 

 tain (except in one or two cases) that their sides converge or have 

 a wedgeform tendency ; so that no estimate can be formed of the 

 depth at which they terminate. In this respect therefore they do 

 not seem to agree with the metallic veins. 



Moreover, I have not observed that they branch off into slender 

 strings, or (except in some very rare instances) that they swell into 

 (what the miners term) bellies, after the manner of the metallic 

 veins. 



The dykes, whether they occur in primitive or secondary coun- 

 tries are nearly vertical. The mean angle of deviation from the 

 perpendicular deduced from nine cases occurring in primitive rocks 

 was 13°, the extremes being 9° and 20°. The same angle deduced 

 from ten other cases was 7° to the N.E. But I am not warranted 

 in drawing any general conclusion as to what point of the compass 

 and in what degree they deviate. The angle of deviation in the 

 two remarkable dykes on Arragh mountain is somewhat consider- 



2 F 2 



