90 STANNIFEROUS DEPOSITS OT TASMANIA. 



of tin, iron, arid silica, which is sufficiently hard to require blast- 

 ing. This is called the north lode, and it can be traced for fully 

 half-a-mile on the ridge of the mount. I regard this as nothing 

 more nor less than a highly ferruginous and stanniferous porphyry, 

 the disruption and subsequent decomposition of which has formed 

 the drift deposit on the side of the basin now being worked. So 

 rich in tin stone is this formation that blocks several hundred- 

 weight of nearly pure binoxide of tin arc broken off. The por- 

 phyry occurs in three different conditions at Mount Bischoff : 

 first, as highly ferruginous, as at the Company's ground ; secondly, 

 as a very felspathic rock, slightly coloured by the presence of 

 oxide of iron, as seen at the adjoining claim of Messrs. Walker 

 and Beecraft, and other sections ; and thirdly, as a light-green- 

 coloured rock, which appearance is due to the presence of olivine. 

 In this latter condition, let me observe, I have found it hitherto 

 poor as a matrix of tin ore. Small grains of native copper have 

 been found associated with the ferruginous tin ore ; while on an 

 adjoining section belonging to the Company rich argentiferous 

 galena obtains. Nothing up to the present time has been done 

 to develop the latter ore. Lodes of sulphides of antimony and 

 zinc exist — the former having a gangue of carbonate of iron 

 andfluor spar, the latter offluor spar alone. 



That Mount Bischoff has suffered much dislocation by eruptive 

 forces is seen by an extensive line of fault running north and 

 south, whereby the whole of the eastern side of the mountain has 

 been depressed, thereby producing a line of vertical cliffs in some 

 parts little less than 100 feet in height, and which afford a fine 

 example of how the eruptive and intrusive porphyry has contorted 

 and transmuted the older sedimentary strata. On the top of this 

 line of cliffs the older palaeozoic slates repose at intervals where 

 the igneous rock has not burst through, while at the base they 

 are again seen having the same strike, which is north 10° east. 



Up to the present time, I may observe, sluicing operations have 

 been carried on by dam water, which, after passing through the 

 sluice-boxes, returns to the reservoir, to be again pumped up. As 

 a consequence, the water is always thick, and much of the fine 

 tin ore is carried away in the tailings. At least 40 per cent, thus 

 escapes. Now, however, that a tramway one mile and a-half long 

 is nearly completed to the Waratah River, which encircles half of 

 the base of the mountain, and where there is a never-failing supply 

 of clear water, and also falls 154 feet high, all the rich tailings 

 will be dressed and crushing-mills be set in operation by the 

 water power. 



The traveller, in ascending Mount Bischoff, passes at a single 

 step from the great basaltic flow occupying the surrounding 

 country to the stanniferous porphyry, the point of contact being 

 most clearly defined. 



