"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.... Here is how it unfolded, followed by a learned correction received in early 2021, offering new insights into the lithology, locality, and historical context.

1. The rock itself appears (I thought) to be 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 (however: see under "lithology" in the concluding update).

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 might, I thought, 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 may or not be a plausible copy of 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!

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.