Quartz sandstone;
Cambrian basal quartzite, Loch Assynt, Scotland
This is a pure sandstone composed of rounded grains of
quartz, cemented by further quartz between the grains. This
makes the rock very hard and resistant to weathering and
erosion, rather like a metamorphic quartzite formed by the
action of heat and pressure. Even though both grains and
matrix are made of the same material, you can see the
rounded outlines of the sand grains in the hand specimen, as
the original particles were thinly coated with iron oxide
dust. There is a very small number of feldspar grains,
showing up chalky white on the rock surface.