Xlll 



and Messrs. Kayser and Provis speak of the quartz porphyry and 

 topaz porphyry. Mr. A. W. Clarke in 1892 wrote a short note on the 

 microscopical appearance of a specimen ot this rock in the collection of 

 Mr. R. L. Jack, Government Geologist for Queensland ; but while 

 recognising a radially arranged mineral, highly coloured between 

 crossed "nicols," failed to recognise it as topaz. Finally, the topaz and 

 quartz porphyries of Mount Bischoff are recorded in the catalogue of 

 minerals by W. F. Petterd, p. 90 (1896). No systematic microscopical 

 examination seems to have been published since von Groddeck's time. 

 His material was plainly limited, and the essential nature and deri- 

 vation of the rock were not dealt with in his painstaking inquiry* 

 After renewed examination in the field, and with the aid of an ex- 

 tensive series of microscopical slides, Messrs. Twelvetrees and Petterd 

 restate the problem, and as they believe advance a step in the process 

 of its solution. In this paper the position is restated at considerable 

 length, and they sum up the petrogriphical conclusions to which their 

 inquiries have led them as follows : — •' I. That the quartz porphyry 

 is not a marginal portion of the main granite mass, but belongs to 

 dykes running through the granite, and having a slightly different 

 composition from the latter. 2. That it partakes of the nature of 

 elvanite with occasionally a quartz felsite facies. 3. That both in its 

 microcrystalline condition and its felsitic modification, it has been 

 subject to topazising and touimalinising agencies of a hydroplutonio 

 nature, which have, when unchecked, transmuted the rock into a 

 topaz-quartz porphyry. (4.) Tnat the crystallisation of the cassiterite 

 was contemporaneous with that of the topaz and quartz. As to 

 whether the tin ore ascended as a fluoride, or stannous acid was derived 

 from the individual components of adjoining rocks, the microscopical 

 appearances convey the impression that the condensation or pre- 

 cipitation took place in the presence of water. (5.) That the great 

 Brown Face workings are not in the basin of a vent issuing from the 

 bowels of the earth, but are in the iron gossan ot a fissured and dis- 

 integrated area enclosed by the quartz-porphyry dykes. Our study 

 lays no claim to be exhaustive. We have approached the subject 

 simply with the desire to record such contributions to our knowledge 

 as may be gleaned from the evidences furnished by microscopical petro- 

 graphy ; and we lay before the Society this essay to expound the nature 

 and genesis of the much debated Mount Bischoff rock, hoping that 

 extended work by others will effect a further advance towards the 

 solution of the problems which are involved in the inquiry." 



Attached to the paper is a list of minerals known to occur in the 

 elvan dykes of Mount Bischoff, being 34 in number. Also an illus- 

 trative plate. 



TASMANIAN ABOEIGINES. 



Mr. J. B. Walker read an interesting paper on the Tasmanian 

 aborigines from notes from the journal of his father, Mr Geo. Washing- 

 ton Walker, and was aided in his explanations by lantern slides, Mr, 

 Arthur Butler manipulating the lantern. He said that in 1832 Messrs. 

 James Backhouse and G. W. Walker, two members of the Society of 

 Friends, arrived in Hobart from England, the object of their visit to 

 the Australian colonies being a philanthropic one. One purpose they had 

 in view was an investigation of the condition of the prisoner population 

 and the working of the penal settlements. Another was to inquire 

 into the treatment of the aboriginals. The reports which they from 

 time to time made had considerable influence in obtaining an amelior- 

 ation of the condition of the large number of men under penal dis- 

 cipline. In 1832 they visited the aboriginal establishment at Flinders 

 Island. The deadly feud between the natives and the settlers, which 



