of the North of Ireland, 231 



la general there Is no foreign matter between the substance of 

 the dyke and the rock it divides, excepting a slight rusty appearance 

 on the surface of the latter. The contact between the two is 

 pretty close, but they may always be disjoined by the blow of a 

 hammer. When the dyke is prismatic, a hollow interval between 

 the two may sometimes be observed. 



Since the average direction of the dykes is from south-east 

 to north-west, and since the average dip of the strata in the north 

 of Ireland is to the south-east, it will follow (independently of the 

 several observations on the bearings of the strata contained in the 

 table) that the direction of the dykes is nearly perpendicular to that 

 of the strata. 



Moreover since the longitudinal vallies and the metallic veins of 

 a district are generally parallel to one another, and to the direction 

 of the strata which they intersect, it will follow that the dykes will 

 cross the longitudinal vallies, and that where metalliferous veins 

 and dykes occur together, that one of the two will cut the other. 

 This is the case at the lead mine of Kildrim in Donegal, where the 

 dyke divides the vein. Similar facts are recounted in the Philoso- 

 phical Transactions, 1790, page 93, by Mr. Mills,* as occurring at 

 the lead mines of Persabus and Glasgow-beg in the isle of Hay j 

 where although the directions of the dykes are not uniform, as in 

 the north of Ireland, yet they cut the veins nearly at right angles. 



•w 



* M. Brongniart as well as Dr. Richardson from whom he quotes, (Irish Trans. Vol. 9, 

 page 22,) appears to have misunderstood this passage, and adduces it to prove that dykes 

 are traTersed by lead veins, the reverse being the case : Traite de Mincralogie, tom.l. p. 462. 

 He also quotes some observations said to be made by M. Humboldt on the basalt of 

 Unkeln, from Journal des Mines, No. 19, p. 378, to prove that dykes are sometimes 

 metalliferous ; but it does not appear on referring to that number that dykes are there 

 spoken of, nor indeed does the reporter appear to have been satisfied with the expression 

 ^' raies metalliques," or to have relied much on the accuracy of the observations. 



