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ance of a basin -like vent at the summit of the Mount is 

 deceptive and suggests false conclusions, for it is due merely 

 to the directions of the several dykes which enclose the 

 ground. The only channels from below are the fissures of 

 the dykes and veins. As the underlay of the dykes is 

 towards the central area, they most likely intersect in depth, 

 and hence the inference suggests itself that, sooner or later, 

 the Brown Face mass will pass down into dyke or lode rock. 

 Geologically these porphyry dykes have been looked upon as 

 protrusions or apophyses of the underlying granite, thrust 

 through the slates, but such microscopical evidence as we 

 have been able to collect tends to show r that their mineral- 

 ogical constitution differs from that of the nearest granite 

 exposures. White mica is not known in the granites of that 

 part of the island, its place being taken by biotite. It is likely 

 that, though these dykes have a granite source, they represent 

 fissures traversing alike the slates and the granite which 

 probably immediately underlie them. 



The nearest exposures of granite rocks occur at four and 

 five miles from Waratah, viz., porphyritic granitite on. the 

 Corinna-road, four miles distant, and granitite at Wombat 

 Hill, five miles. The porphyritic ingredients of the former 

 are orthoclase aud plagioclase felspars, dark brown and light 

 magnesian micas and quartz in an allotriomorphic ground- 

 mass of quartz, orthoclase and mica; consequently there is a 

 repetition of all three minerals. The most frequent minerals 

 are the mica and felspar, while quartz is the least abundant 

 porphyritic constituent. The mica is rich iu inclusions of 

 apatite ; grains of zircon in ground-mass. 



The Wombat Hill granitite consists of orthoclase + plagio- 

 clase + biotite + a little green hornblende. There is a dis- 

 position to pegmatitic intergrowth of quartz and felspar. 

 The biotite encloses quartz, apatite, and zircon, and in colour 

 is dark brown, sometimes bleached out. Near the 7-mile 

 peg, on the Waratih-Corinna-road, a limited quantity of 

 alluvial tin has been worked. The drift consists of quartz 

 and black tourmaline of the more abundant type, which is 

 unknown in the Mt. Bischoff elvans. 



To sum up, the petrographical conclusions to which our 

 inquiries have led us are : — 



1. 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 occasion- 

 ally a quartz felsite facies. 



3. That both in its micro-crystalline condition and its 

 felsitic modification it has been subjected to topazising aud 



