Lapiés and Solution Pits in Olivine-Dolerite Sills at Slieve Gullion, Northern Ireland

Abstract

Evidence is presented to show that deep flutings and cavernous forms, occurring on essentially vertical joint planes of olivine-dolerite sills on Slieve Gullion, resemble the lapids of Hawaii and New Zealand. They have been formed in a similar way, that is, as a result of chemical erosion of basaltic rock by surface waters escaping down the joint planes of essentially impervious layers. Occasionally the lapiés intersect vertical sheetlike veins of microgranite, which, on account of their greater resistance to erosion, remain like partitions up the middle of lapiés. A hypothesis that the microgranite was intruded while the dolerite was still fluid has been based on the erroneous suppositions (a) that the lapiés were originally occupied by microgranite with a pipelike form and (b) that this form is inconsistent with the microgranite having been em-placed in solid dolerite. Structural criteria are therefore established by reference to which it can be discovered whether microgranite, irrespective of whether it occurs as sheetform veins or as pipes, was emplaced (a) in molten or rheid dolerite or (b) in crystalline and rigid dolerite.