Distinctive features:  
Poorly sorted dark grey to greenish, fine-grained sandstone.

Colour: 
Various sahdes of dark grey to dark greenish-grey.

Texture and granularity: 
Granular, fine-grained.

Composition: 
Quartz, plagioclase and tiny rock fragments set in a matrix of microscopic quartz, feldspar, clay and other minerals that are too small to determine without a microscope.

Field associations: 
Formed at bottom of ocean trenches bordering continents by avalanches of submarine origin. Occurs in association with black shales of deep sea origin.

Varieties:
Feldspathic greywacke, which is rich in feldspar; and lithic greywacke, which is rich in tiny rock fragments.

 

Worldwide, but especially bordering ancient fold mountain ranges.

 

None of any importance.