BY W. F. PETTERD, C.M.Z.S. 35 



of the felspar crystals can be distinctly traced, the whole 

 being converted to tin ore, and the shape of the crystal 

 being faithfully preserved. At the Stanley River the 

 imbedded felspars have also been converted to cassiterite 

 and green tourmaline, and at Mt. Bischoff the same thing 

 has occurred with respect to the felspars originally con- 

 tained in the topaz-porphyry so characteristic of the 

 locality. The replacement at this spot is not always com- 

 plete, inasmuch as the interior of the felspar outline may 

 be but partially filled with the black crystals of cassiterite, 

 but the whole stands out prominently on the white ground- 

 mass. At Cope's Creek, New South Wales, masses of 

 fossilised wood have been obtained in the older drifts 

 which are overlaid by basalt, partially converted to tin 

 ore. At North-East Dundas breccias, consisting of frag- 

 mentary rock, probably broken from the walls of a lode or 

 fault, have been obtained, absolutely cemented together 

 with cassiterite, which has doubtless been deposited from 

 circulating waters. The same phenomenon has been recorded 

 as occurring at several localities in Cornw^all, England. 

 "It is contended by the chemist Bischoff that ordinary 

 carbonated waters coursing through tin-bearing rock might 

 be competent to dissolve felspar and to deposit tinstone in 

 its place " (Rudler, " Minerals of the British Islands," 

 1905). The same remark probably applies to the other 

 transmutations referred to. 



Referring generally to tin-bearing veins or lodes, the 

 formation or origin of these is the result of such a com- 

 plex process that there is room for several explanations as 

 to the different phases of cause and development. At 

 Mt. Bischoff, which geologically stands out conspicuous as 

 being almost unique, the cassiterite occurs disseminated 

 in comparatively small crystals as well as in the massive 

 form in a huge elvan course or dyke composed of quartz 

 and topaz-porphyry (topazised quartz porphyry), which 

 doubtless extends upwards from a deep-seated mass of 

 granite, into the surrounding contorted and disturbed 

 Cambro-Ordovician slates, or " killas," as the rock would 

 be termed in Cornwall. It has attendant lodes of various 

 sizes and strike, one at least of which intersects the main, 

 or, as it is termed, the " Queen " dyke. These attendant 

 lodes are in the main composed of quartz-porphyry, and so 

 far as known only one shows any conspicuous departure in 

 composition. The gangue of this is apparently a greisen 

 unusually rich in white mica, much of which, so far as 

 exploited, is decomposed and stained by iron oxide. Other 



