126 



tourmalinising agencies of hydro-plutonic nature, which have, 

 when unchecked, transmuted the rock iuto a topaz quartz- 

 phyry. 



I. Thai the crystallisation of the cassiterite was contem- 

 poraneous 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 precipitation 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 of a fissured and disintegrated area 

 enclosed by the quartz-porphyry dykes. 



Our studv lavs no claim to he exhaustive. We have ap- 

 proached the subject simply with the desire to record such 

 contributions to our knowledge as may be gleaned from the 

 evidences furnished by microscopical petrography, and we 

 lay before the Society this e.-say 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. 



List op 34 Minerals known to occur in the Elvan 

 Dykes of Mount Bischoff. 



Apatite — Occasionally obtained in small crystals, which can 

 be recognised with unaided vision. 



Arsenopyrite — In considerable abundance in the lower levels, 

 associated with other forms of pyrites, through the 

 mass of which are scattered small crystals of 

 c.-issiterite. 



Arsenic, native — As narrow blades aud patches between the 

 lamina3 of siderite, nuorite, and pyrites in lower 

 level North Valley workings. 



Azurite — Occasionally met with in bunches of minute crystals 

 in the Brown Face with malachite. 



Cassiterite — The pyramidal crystals are often beautifully 

 formed, with complex terminations and macles. 

 The colour is invariably intensely black. 



Chalcopyrite — The massive form only known in limited 

 quantity. 



Copiapnte — As an efflorescence in the older adits. 



