80 BASALT FAMILY. The fragments are indeterminate angular, and rather ’ sharp-edged. It occurs in distinct concretions. These are generally columnar, varying from a few inches to several fathoms, even to upwards of 100 feet in length : the number of sides varies from three to nine, and of these the nine sided are the rarest; they are straight, bent, and either parallel or diverging; sometimes they are articulated, and the joints have concave and convex faces. In moun tains, these concretions are collected into large groups, and many of these groups or colossal concretions form u hill or mountain. Sometimes it occurs in tabular, some times in globular concretions; these latter are frequently composed of concentric lamellar concretions, or of co lumnar concretions radiating from the centre. Some varieties are composed of large, coarse, and fine granular concretions. It is opaque, or feebly translucent on the edges. It yields a light grey-coloured streak. It is semi-hard, bordering on hard. It is rather brittle. It is difficultly frangible. Specific gravity, 3.065, Klaproth. 3.082, Karstcn. Chemical Characters. Before the blowpipe it melts easily, without addition, into an opaque black-coloured glass. According to Dr Kennedy, the basalt of the Castle Rock of Edinburgh softens at 45° of Wedgwood; that of Staffa at 38° ; and I obtained similar results with the basalts of Arran, as is mentioned in my mineralogical description of that island Constituent * Vid. Mineralogical Travels, vol. i.
