Brecciation-Hydraulic

This type of brecciation occurs when hydrothermal fluids within a fracture exert terrific pressure on the walls of the fracture, which pushes the walls apart If the contained fluid finds an escape route there is an intantaneous release of pressure on the fracture walls, which shatter and implode fragments of wall rock into the fluid. The fluid eventually cools and solidifies, creating examples such as this specimen. The shattered fragments don't move a great distance as can be observed in the inset picture. This specimen was collected from Dylife mine in Powys.

Sole marks, Oscillation ripples.

These structures are a sub division of groove casts, oscillation ripples. Found in a shoreline or shallow marine environment, they are the result of two directional currents. The cross section is not immediately obvious, but my close inspection of the specimen as a whole is conclusive. The specimen was collected from Tan y Foel quarry, 5 Km. S.W. of Llanllugan, near Newtown, and is from the Penstrowed grits formation in the Silurian

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.