Porphyritic volcanic tuff

- from Dyfed, southwest Wales


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Figure 1. A curious souvenir, clearly a paperweight, composed of an attractive ornamental stone with a medium polish, green baize beneath, and a sculpture of a stone age structure made out of a pale metal, perhaps tin. There is a story here --- but what is the rock, where is it from, and what does the little sculpture represent?


"Rock of the Month #160, posted for October 2014" ---

A Little Mystery, and the Value of a Reference Collection

The paperweight belonged to the late Wilma Wybourn (née Roberts), a dear friend, to whom this little tale is dedicated with affection. Wilma was of Welsh descent, and had numerous memorabilia from that fair country. That, and some personal history with the Welsh countryside, proved enough to craft an explanation which seems plausible. There are three parts to this mystery....

1. The rock itself is a coarsely feldspar-porphyritic fragmental volcanic, a striking unit of volcanic tuff. Each of the creamy-white feldspars appears to be an aggregate of several smaller crystals, so the term glomerocryst may be used.

The complex Welsh coastline includes a wealth of geological interest, around the coast from Pembroke past Haverfordwest, St. David's and Fishguard to Cardigan (Ordnance Survey, 1964, 1965). These include volcanic and volcaniclastic rocks of early Ordovician age in the Fishguard area (Bloxam, 1971). There are fossiliferous shales with graptolites, volcanic and intrusive igneous rocks, and more. Arenig volcanism is represented by the Trefgarn (Treffgarne), Sealyham and Skomer (Skomer Island) volcanic series (Neville George, 1970, pp.23-25).

2. The origin of the rock is thought to be the Treffgarne quarry, a source of green, purple and black tuff of Arenig (lowest Ordovician) age. Treffgarne is near the coast in what was northern Pembrokeshire and Cardigan, now combined with Carmarthen in the region of Dyfed. Samples were collected at the quarry in summer 1973, and the ornament appears similar to the green variant, albeit more spectacular due to the especially coarse phenocrysts. The Treffgarne rocks show evidence of waterlain bedding, ripple marks and mud cracks, all evidence of a subaqueous environment, whether marine or in a volcanic crater lake (Bloxam, 1971).

Back to the paperweight for a minute: the metal is slightly magnetic, but its magnetic susceptibility cannot be measured by my trusty SM-30 meter, because the metal is evidently a reasonable conductor of electricity (the test leads to a very large negative value, which is not a meaningful result!). The tuff gives a value of 0.3x10-3 SI units, which is not appreciably magnetic.

As an aside, three mines in the Dolgellau gold belt account for the bulk of historical production in the UK, roughly five tonnes of the metal, mostly mined between 1870 and 1910. The gold was in quartz veins cutting Cambrian to Silurian sediments. Curiously, elevated gold values are reported in argillites at Treffgarne (Cameron, 1992).

3. The Stone Age structure is a dolmen, the original constructed of four large pieces of rock. A famous example is located within a few km of the quarry, albeit with 3 "legs" rather than four. The Pentre-ifan Neolithic chambered barrow or portal dolmen lies on a hill north of the Preseli Mountains, overlooking the sea at Cardigan Bay (Ordnance Survey, 1965). Another local example is the Carreg Coetan portal-dolmen monument (Taylor, 2005, p.112). Morphologically similar Neolithic structures are widespread in the world, including Switzerland (Sauter, 1976); south India (Wyckoff, 1951); and the ancient metallurgical districts of southwest Spain, which includes the Iberian Pyrite Belt (Rothenberg and Blanco-Freijeiro, 1981).

The region has many scattered occurrences of volcanic rocks, such as the Asaphus Ash (as at Meidrim), named for a local trilobite, as well as shallow and deeper intrusive igneous rocks (e.g., dolerite [diabase] from Solva Harbour and the Llanwnda gabbro in the Henner Cross area. The Turnstone reference collections include examples of all these, and recognition of the dolmen enabled rapid comparison with local rocks. The paperweight rock may not be from Treffgarne, and the dolmen is evidently not Pentre Ifan, but it seems very likely that the original stone structure, the rock and the paperweight all come from within 20 km of Treffgarne. A nice souvenir of ancient Welsh history!

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Figure 2. Four samples from Treffgarne quarry, representing green, purple and black tuffs. All these rocks are fragmental, as opposed to lava flows of more coherent, crystallized and/or vitrified magma. Selected from samples 112.3-9, collected 22 August 1973.


References

Bloxam,TW (1971) Haverfordwest, Strumble Head and Abereiddy Bay. In `Geological Excursions in South Wales & The Forest of Dean' (Bassett,DA and Bassett,MG, editors), Geologists' Association, South Wales Group, Cardiff, 267pp., 199-205.

Cameron,DG (1992) United Kingdom's gold potential. Northern Miner 78 no.8, 12, 27 April.

Neville George,T (1970) British Regional Geology: South Wales. IGS / HMSO, 3rd edition, 154pp.

Ordnance Survey (1964) Cardigan. Ordnance Survey one-inch map 139, 1:63,360 scale.

Ordnance Survey (1965) Fishguard & Pembroke. Ordnance Survey one-inch map 138/151, 1:63,360 scale.

Rothenberg,B and Blanco-Freijeiro,A (1981) Studies in Ancient Mining and Metallurgy in South-West Spain. Institute for Archaeo-Metallurgical Studies, Institute of Archaeology, University of London, 320pp.

Sauter,M-R (1976) Switzerland: From Earliest Times to the Roman Conquest. Thames and Hudson Ltd, 208pp.

Taylor,T (2005) The Time Team Guide to the Archaeological Sites of Britain & Ireland. Transworld Publishers / Random House Group Ltd, 320pp.

Wyckoff,CC (1951) Kodaikanal: 1845-1945. London Mission Press, Nagercoil, 3rd revised and enlarged edition, 78pp. plus 2 maps.

Graham Wilson, 13-14 September 2014

Visit the Turnstone "Rock of the Month" Archives!

Other samples from this region may be seen here: see Rocks of the Month 53 (halleflinta from St. David's) and 111 (graptolites from Abereiddy Bay).