L331 Sandstone, wacke

Dark grey rock, mainly fine sand.
Does not react with acid, probably mainly quartz. 
Some glints of mica. 
Cement reacts with acid, so it is at least partially carbonate. 
Band of brown-grey material is due to weathering.

The thin-sections are only cut from dark grey, unweathered region of the rock. They are best viewed under high magnification.

Sub-angular to sub-rounded quartz grains. Mainly fine sand, with a significant proportion of silt. >15% clay, so this is a wacke.
A few white mica flakes and opaque grains.
The main cement is carbonate, though the grains are also partially cemented by the clay grade grains.

Wackes are commonly produced by turbidity currents, though we cannot be sure without seeing graded bedding.

sandstone
wacke