A small, elongate plutonic mass cropping out on the southern flanks of Slievenagriddle [J522 449] was first mapped by Traill and Egan (1871) as a syenite. By analogy with a dyke of similar composition, described by Kennedy and Read (1936) from Galloway in Scotland, Patterson (1942) reclassified the intrusion as markfieldite and included detailed petrographic and geochemical analysis with discussion on its mode of emplacement. He recorded vertical contacts with the country rock from its eastern and western margins and concluded, from the occurrence of further gently inclined contacts within the intrusion, that the exposure represents the roof of an igneous stock, implying a more extensive mass at depth. In addition he noted a mineralogical affinity with the younger series of lamprophyre dykes in the region (Cameron, 1977).
Cameron (1977) noted the presence of two further stocks in the vicinity cropping out 500m to the east. The larger of the eastern intrusions is macroscopically identical to the western stock and was also identified as markfieldite. The third and smaller markfieldite body crops out 90m to the ESE. Cameron (1977) noted that the group of intrusions has a markedly linear east-west trend and suggested that the separate outcrops appear as members of a single dyke system, itself a constituent of the younger series of the lamprophyric swarm, and occupying a major younger series tension fracture trending 20° clockwise to the regional strike.
