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quartz rock, which consists of a granular quartz ground- 

 raass enclosing large phenocrysts of quartz (often with 

 striking marginal zonal inclusions), but containing no topaz. 



Tourmaline is here a rock-forming mineral, making a dense 

 green stone, which at one time was thought to be chlorite. 

 It is arranged in microscopical divergent, and felted prisms, 

 and needles, bluish green in colour. Occasionally, rectangular 

 crystal forms, which can be no other than those of orthoclase 

 felspar, are discerned filled with or composed of small rods of 

 tourmaline, demonstrating absolutely the secondary nature 

 of this mineral. In the ground-mass of this tourmaline rock 

 are nests of prismatic and granular topaz. Sphene is 

 plentiful. 



We have had slices cut showing the junction of the 

 porphyry with the slate. The boundary between the two 

 is perfectly sharp and well defined, and even where' frag- 

 ments of slate have been torn oh and surrounded by porphyry 

 the amount of contact alteration is inconsiderable. 



The Mt. Bischoff stanniferous ground has always been 

 looked upon as unique. It has been compared with the 

 occurrence of the topaz quartz-porphyry of the Saxon 

 Schneckenstein ; but the latter, though presenting micro- 

 scopical structural resemblances, differs widely in its 

 geological relations, being a brecciated dyke in the con- 

 tact zone of the tourmaline granite. The rock of the 

 Saubachschlucht dykes in that locality shows a startling 

 resemblance to the Mt. Bischoff porphyry, both in hand 

 specimens and under the microscope. It is a quartz-porphyry 

 dyke rock, with porphyritic quartz and felspar in an 

 allotriomorphic quartz ground-mass. Secondary muscoviteor 

 talc is diffused through the rock, and topaz has crystallised 

 as rods in the porphyritic felspars. There is considerable 

 difficulty in determining the exact nature of the ground 

 which is being worked at Mt. Bischoff. Some think it is a 

 stockwerk ; others a thermal spring deposit. Another 

 theory is that it is the silicious plug of a vent in which 

 stanniferous vapours ascended and condensed. Further 

 work and study on the spot, especially underground, are 

 requisite before a wholly satisfactory hypothesis can be 

 framed. The results of microscopical investigation and our 

 examination in the field lead us to look upon the mass of the 

 great Brown Face as disintegrated, silicious, ferruginous, 

 partly detrital rock surrounded by dykes on three sides, and 

 fissured and affected by the agencies which formed the dykes. 

 The concentrated material shed by the overhanging dykes 

 h:is probably enriched the ground, while the mere disinteg- 

 ration of the enclosed area itself and consequent lowering of 

 its surface would work in the same direction. The appear- 



