Plane polarised light. Greywacke - graywacke - GR: NW999538, on coast just S. of Portpatrick, Galloway, Southern Uplands, Scotland. Ordovician (Caradoc-Asgill). A typical greywacke is poorly sorted and consists of abundant finegrained clasts, showing up as almost opaque when viewed under plane polarised light.
The ill-sorted quartz, feldspar, lithic clasts and other mineral grains in a clay matrix are probably the result of turbidity currents and this rock is also known as a turbidite.

Under Cross polarised light the rock can be seen to consist mainly of ill-sorted angular to subangular quartz and feldspar grains within a groundmass of  clay minerals and some carbonate; the latter is visible just right of centre towards the top. At the near centre is a sub-rounded fragment of plagioclase feldspar with twinning visible. It is 0.2mm in width.

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