In 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 Philosophical Transactions, 1790, page 93, by Mr. Mills,[1] as occurring at the lead mines of Persabus and Glasgow-beg in the isle of Ilay; 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.
- ↑ 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 traversed by lead veins, the reverse being the case: Traité de Mineralogie, tom. 1. 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.
