Eclogite, Norway.

Eclogite is an uncommon rock. It typically results from the high pressure metamorphism of mafic igneous rock such as basalt and gabbros at depths of 50km. or more in the mantle or lower crust. It is composed of mainly two minerals, reddish brown Almandine Pyrope garnet, and the green sodium rich pyroxene Omphacite. Isotope dating of the Norwegian Eclogites in the 1970, 80s, indicate that they are early Palaeozoic ( 450-400 million years ), suggesting that they were formed during the Caledonian orogeny.  This brilliantly colorful  specimen is from Almenning, Northern fjords in Norway, and was a gift to me by three members of the club, Michelle, Eleri, and Janey. following my traumatic experience of severe cellulitis, and a lengthy stay in hospital.

Obsidan, New Zealand.

Obsidian has earned the popular name of "volcanic glass" for it's resemblance to glass. It has similarities to the appearance of glass because of a high silica content that is between 60% and 85%. Obsidian is classified as a rock. and occurs when silica rich ejected magma rapidly cools. This specimen originated from New Zealand.  The conchoidal fracture is the result of rapid cooling prohibiting the development of a crystal structure which would have cleavage planes. This type of amorphous material is known as a mineraloid. With the passage of time the unstable obsidian can begin to crystallize and radial clusters of cristobalite can form at random in the obsidian, thus creating the attractive "snowflake obsidian. The word cochoidal is derived from the description of a shell " conch", semicircular.

Flysch. Swiss Alps.

This specimen was a gift from Bill Fitches who collected it from the Salanfe valley in the Martigny area near the Swiss-France border in the Swiss alps. His description of the specimen is as follows. The Flysch is a suite of marine sedimentary rocks of Eocene age, which was deposited in the Tethyan ocean between the African and European continents just before they collided to create the Western Alp mountain chain. They are low energy  turbidites composed of thinly bedded mudstone and siltstone. The sample  has many veinlets of light coloured carbonate , probably calcite, which cut the bedding at right angles. They were produced during the plate collision and the rise of the Alps. The sample was squashed perpendicular to the bedding planes, and strongly extended along the bedding. This deformation caused lots of cracks which filled with carbonate segregating from the host rock to make the arrays of veins. The veins are in several directions, implying that the direction of extension changed during the deformation history.

Wrekin quartzite, Ercall quarry.

The Wrekin quartzite is exposed in Ercall quarry, Shropshire, from where this specimen was collected  The quartzite is composed of  fine pale grey quartz sand, consolidated by siiceous cement. It is lower Cambrian in age, and the quartzite is resting unconformably on the Uriconian volcanics.

Concretion. Trewern,Welshpool.

This concretion has a definite sharp edge around the broadest part, almost as though it is two halves stuck together. I am not absolutely sure, but I think it is of the type described as "cone in cone". The only way to be sure would be to break it open, which I am loathe to do. Concretions are fairly common in the Welshpool area, and I have heard stories that they were commonly used as doorstops. When it has been seen by young children they thought it was a dinosaur egg. It was collected many years ago at Dingle farm, Trewern, Nr. Welshpool in central Wales. There is a comprehensive explanation of concretions by Tony Thorp in issue No 1 of the Silurian magazine which can be accessed on this site.