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Updated: May 22, 2025
Obsidian is of a black or ash-grey colour, and though opaque in mass is transparent in thin edges. Among the rocks of the trachytic family, or those in which the feldspars are rich in silica, that termed Clinkstone or Phonolite is conspicuous by its fissile structure, and its tendency to lamination, which is such as sometimes to render it useful as roofing-slate.
In part it approaches in character to a green and blue roofing-slate, while part is extremely quartzose, the whole mass passing downward into micaceous schist. The vertical section here exhibited is about three feet in height, and the layers are sometimes so thin that fifty may be counted in the thickness of an inch. Some of them consist of pure quartz.
Beds of pure quartz also occur in this formation. In some districts, garnets in regular twelve-sided crystals form an integrant part of mica-schist. This rock passes by insensible gradations into clay-slate. This rock sometimes resembles an indurated clay or shale. It is for the most part extremely fissile, often affording good roofing-slate.
The third or lowest division south of the Grampians consists of grey paving- stone and roofing-slate, with associated red and grey shales; these strata underlie a dense mass of conglomerate. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitchell in beds of corresponding age in Perthshire; and Mr.
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