The difference between schists and gneisses is mainly that the latter have less highly developed foliation; they also, as a rule, are more coarse grained, and contain far more quartz and felspar, two minerals which rarely assume platy or acicular forms, and hence do not lead to the production of a fissile character in the rocks in which they are important constituents.
Hence sands are more coarse grained than clays.
The soap is from this again grained off or salted out, and the underlye so thrown down carries with it coloured impurities which may have been in the materials or which arise from contact with the boiler.
They are dark blue-grey, fine grained and durable, and are much used for flagging and kerbing and for sills, caps and steps.
The wood is fine grained and satiny.
