Term: basalt (mining)
One of the common trap rocks. It is composed of Augite and feldspar, is hard, compact and dark green, or black, and often has a regular columnar form.
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).]