Rhyolite is a silica rich extrusive igneous rock. These specimens were collected from Ercall quarry in Shropshire on a field trip in July 2001. They were picked out of the rubble at the foot of the quarry face, which was plentiful at the time. One of the specimens can be described as flow banded. Unusually, this picture was taken using a photocopier before digital camera was the norm.
This specimen of dolerite ( diabase ) was collected on a field trip to a local quarry in June 2001. The Criggion quarry, on the outskirts of Welshpool in Powys is well known for supplying very hard stone, mainly used for road surfacing. The rock type is described as an albitised olivine diorite. The mineralogy and appearance of the rock indicates a degree of chemical alteration due to reactions between original minerals and migrating fluids. The usual appearance of lath shaped plagioclase crystals is absent. Instead the dolerite has a spotted appearance. This assumption is due to my observation, and therefore a better description might be obtained by a laboratory investigation. Note the difference between weathered surface and exposed surface.
The specimen was collected from debris on the beach at Blue Anchor bay on the Somerset coast. The occasion was a field trip organised by the club in June 2014. The mudstones here are rich in nodules of pink alabaster ( fine grained gypsum ) hydrous calcium sulphate, a secondary product of the original Triassic evaporites which were deposited in the first instance as halites from drying up salty playa lakes. The beach was littered with quite a number of large boulders, some as large as 2 metres that were detached from the cliff by erosion, and with the same characteristics as the specimen.
The name tonalite is derived from a type locality rock, adjacent to the Tonale line, which is a major structural lineament in the Alps. This specimen was collected on a field trip to Coed-y-Brenin in the Snowdonia national park, North wales. Tonalite has the same petrological significance as andesite, but has remained at depth, crystallising under plutonic conditions, allowing crystals to grow to a reasonable size, and evolving chemically in a felsic sense. Tonalite is composed of essential quartz, Plagioclase feldspar, mafic minerals ( biotite, pyroxene, hornblende ), and minor alkali feldspar. The inserts show crystals of pargasite, which have been exposed on a fractured surface. A full account of the field trip to Coed-y-Brenin in November 2020 can be found on this website under the heading of past news.
Concretions are very common in the mudstones of the Welsh basin. Most of them are found without an apparent mineral content. There are exceptions with an occasional calcite core. This specimen, which has an infilling of calcite is the only septarian concretion that I have observed in central Wales. It was discovered at Abermule, very near to the river Severn, 6Km NE of Newtown. This area is on the outer margins of the Welsh basin, and septarian concretions are known to form at relatively shallow depths of water and sediment.