Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/235

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

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 isolated 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 ascertain (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 countries 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 considerable,