Term: trap, or trapean rocks (mining)
Ancient volcanic rocks, composed of feldspar, hornblende and augite. Basalt greenstone, amygdaloid and dolomite, are trap rocks.
View pictures relating to mining at Wisconsin Historical Images.
[Source: "Glossary of Miners' & Mineral Terms" in St. John, John R. A True Description of the Lake Superior Country ... (New York, W. H. Graham, 1846).]