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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. |
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. |