Basalt
a word said to have been derived from an African word, and to have meant basaltoid syenite, from Ethiopia or upper Egypt. In general the name is given to any trap rock of a black, bluish, or leaden gray color, and possessed of a uniform and compact texture.
In a special sense it is a trap rock consisting of augite, feldspar and iron intimately blended, olivine also being not unfrequently present. The augite is the predominant mineral; it is, sometimes, however, exchanged for hornblende, to which it is much akin. The iron is usually magnetic, and is, moreover, often conjoined with titanium. Other minerals are also occasionally present, one being labradorite. It is distinguished from doleryte or dolerite by its possessing chlorine disseminated through it in grains. It is of a very hard, endurable nature, and may be used to advantage in macadamizing roads.
The specific gravity of basalt is 3.00.
There are fine columnar basalts at the Giant's Causeway in the N. of Ireland; in Scotland at Fingal's Cave and other parts of the Island of Staffa; and along the sides of many hills in the old volcanic district of western and central India.
At West Orange, N. J., the face of the First Mountain exhibits basaltic formations, the lines being diagonal and suggesting a huge open fan, with its ribs converging near the ground.