|Tievebulliagh, Co.Antrim: view from north.|
|Porcellanite associated with Tertiary dolerite plug|
|Tievebulliagh hill (height 1346 feet) is located at the eastern edge of the Antrim basalt scarp 4km west of Cushendall village. Access is via a farm track which leads up to the plateau about 0.5km north of the summit area. Vehicles can be taken to the second gate, where they may be parked on the grass verge (if conditions are dry). From here a track, unsuitable for cars, branches to the south and contours to the base of the hill scarp where scree material has recently been excavated presumably for fill purposes. Dark-coloured, angular fragments of porcellanite are common in this scree and in a smaller one to the south: most of these fragments were produced by a Neolithic axe factory at this site.|
|The source of the porcellanite can be found in outcrops at the top of the screes, and also the contact between the basalt country rock and olivine dolerite forming an inclined volcanic plug. |
Porcellanite occurs as a large block (5.5m by 1.7m currently exposed) within a basal layer of olivine-rich dolerite (picrodolerite). This block was originally of Interbasaltic Formation lithomarge, which foundered into the doleritic magma filling the volcanic plug. Porcellanites of varying mineralogy and texture were formed from laterite and lithomarge of varied composition. Colour variations are a guide to the mineralogy: white patches are largely of mullite, yellow ones are rich in corundum, brown streaks are rich in pseudobrookite, and black patches contain hematite and magnetite. As the hornfelsed rocks are fine-grained, a petrological microscope is needed to identify other unusual minerals which are present including sapphirine and the high-temperature silica polymorphs, cristobalite or tridymite.
|Tievebulliagh, Co.Antrim: porcellanite outcrop.|
|Tievebulliagh is fully described in Key Site 14 - Tievebulliagh, within the Igneous/Tertiary Block. Its inclusion in the Mineralogy/Metallogenesis Block is in recognition of the special interest attached to the mineralogy of the porcellanite, which was first described in detail by Agrell and Langley in 1958. A summary appears in Emeleus and Prestonís field excursion guide (1969). As noted in Key Site 14, the metasomatic rocks and picrodolerites are worthy of further research, which could add further minerals to those already described. Recently, detailed geochemical studies of Tievebulliagh porcellanite have been undertaken by Joanne Curran, Ian Meighan and co-workers in order to distinguish between porcellanite axes from this locality and from Brockley on Rathlin Island (Curran et al., 1997; 2000).|
Tievebulliagh is an important archaeological site.
|See also Key Site 14 - Tievebulliagh and Key Site 435 - County Antrim Karst; Tievebulliagh area.|