94 



My attention has been drawn to a paper on tlie geology of 

 the Zeehan and Duudas silverfield, read Nov. 20, 1895, before 

 the Institution of Mining and Metallurgy at the Jermyn- 

 street Museum in London, by Mr. W. F. Thomao, who has, I 

 notice, correctly diagnosed this white rock as a mela- 

 phyre, and refers to its occuireuce as favourably affecting the 

 richness of the silver-lead lodes near Zeehan. 



III. PicRiTic Basalt from Mount Horror. 



There is a small class of rocks composed of the ferro-mag- 

 nesian silicates and containing little or no felspar. These are 

 the ultra-basic rocks, the specific gravity of which is higher 

 than 3, and as high as 3*2, 3 "3. By the addition of a 

 felspathic constituent and its increase the specific gravity is 

 reduced. Olivine no longer occupies an exclusive place, but 

 augite and hornblende come in and eventually become the 

 dominant ferro-magnesian silicates. When this happens the 

 rock becomes a picrite, augite, or hornblende-picrite, as the 

 case may be, and the picrite itself is liable to go on vaiying 

 in the same direction until the augite-picrite may finally 

 merge into olivine dolerite, or even into olivine basalt. 



In Tschermak's original type rock picrite (1869) olivine 

 constituted one-half of the rock, the remaining constituents 

 being diallage, hornblende, and biotite. Griimbel described 

 a few years later a palaeopicrite from the Fichtelgebirge, an 

 olivine-augite rock. In 1880 Dr. (now Sir A.) Geikie 

 described augite-picrites from Scotland, in Inchcolm and at 

 Blackburn, the latter with glass and abundant felspar, being 

 a plagioclase-augite-olivine rock. Prof Judd, in 1885, pub- 

 lished observations on picrites from the island of Eum and 

 the Shiant islands. We need not here concern ourselves 

 with the extension of the name to the hornblende-picrites by 

 Prof. Bonney, but restrict our remarks to the augite division 

 of the group as represented by a rock from Mt. Horror in 

 Tasmania. 



The rock is a dark looking, almost black, crystalline stone, 

 with large crystals of augite porphyritically set in the matrix, 

 with smaller grains of olivine visible with a hand magnifier. 

 Its weathered surface is rusty brown, with numerous pro- 

 truding augite crystals. It is excessively tough, and its 

 weight when taken in the hand prepares one for its high 

 specific gravity. Determinations by Walker's specific gravity 

 balance gave' for different samples 3-11, 3-14, 3'16, 3163, 

 3-169. To the last specimen I applied the liquid process, 

 using Klein's tungsto-borate of cadmium solution of 3*28 

 density. Floating the fragments upon this dense fluid and 

 diluting it until they remained suspended in any given 



