wwh: I performed a search on Google and entered the search like this:

"Giant's Causeway" trap

With what came up, I found the following information about the trap, a type of volcanic rock, of which the causeway, it turns out, is formed:

PHYSICAL FEATURES
As regards minerals in the county, the sub-soil is basalt or trap, which forms the Giant's causesway on the north coast, clay-slate and limestone; there was coal at Ballycastle, and salt mines near Carrickfergus; and iron ore in the hill region extending from Larne to Cushendall <../Glimpses.htm>. The ore was shipped from Larne, Glenarm, Carnlough and Red Bay to the ports of Cumberland, Wales and Clyde. There are numerous large bogs in the county.

http://www.from-ireland.net/descrs/antrimdescr.htm

Best regards,
WordFriend