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Sub-Arkose

111 and 112 show a slightly porous sediment in which the pores have been impregnated with a blue dye. Most of the grains are quartz, elear in plane-polar-ized light (111), and shades of grey with crossed połars (112). Many show uneven interference colours especially at the bottom of the figurę. This indicates that the grains would go into extinction over a rangę of several degrees rather than all at once. This phenomenon, known as undulose extinction, is found in ąuartz from many igneous and metamorphic sources. The sediment also contains significant feldspar. These grains are most easily recognizable in plane-polarized light (111) where they are cloudy and show signs of alteration. Partial solution of the feldspar is well shown by the porous areas filled with blue dye. A little clay showing bright interference colours is also present. A quartz-rich sediment, but with morę than 5% feldspar classifies as a sub-arkose.

111 Sub-arkose in planc-polarized light. Locality: Millstone Grit, Upper Carboniferous, Northern England (x 9).

112 Sub-arkose with crossed polars. Locality: Millstone Grit, Upper Carboniferous, Northern England (x 9).

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