Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/195



Near the Giant's Causeway it is confined to the lower portion of the cliffs, where beds of it in various states are seen, sometimes assuming the character of a decided red ochre, sometimes variegated red and cream yellow, containing imbedded heterogeneous portions, wearing at a small distance the external appearance of agates, but when examined found to be as tender as the softest steatite. Specific gravity 1,92.

Similar characters apply to the bole in other places. The lowest bed of the trap formation where it comes in contact with the upper surface of the chalk, and contains chalk flints imbedded in its mass, is most generally a red ochrey bole.

7. Wacke.

This rock appears to be of extremely rare occurrence in the Ulster trap series; it is found however in Portnoffer near the Causeway, underlying the upper stratum of columnar basalt, and alternating with the wood coal described in article 9; the thickness of the whole amounting to eight feet: it forms vesicular concretions of a trapezoidal or rounded figure; without coherence; texture dull and earthy, with a few specks of mica; the colour varies from yellowish grey to brownish grey, apparently derived from iron ochre. The bed or horizontal dyke of wacke traversing the chalk cliff at Murlough bay has before been noticed. A seam of wacke, not more than three or four inches thick, is interposed between the greenstone and sandstone of Cross hill.