88 STANNIFEROUS DEPOSITS Of TASMANIA. 



by veins and strings of quartz. In Dot a few instances the lime- 

 stones are metalliferous, bearing veins of argentiferous galena, 

 associated with pyrites of tin, iron, and bismuth. At a distance 



of less than one hundred yards inland these strata are covered 

 up by basalt, which, as a rule, presents a beautifully prismatic 

 structure, the columns being made up of pentahedral and hexahe- 

 dral septse with well-defined facets. On leaving Ihe coast for the 

 tin mines, the older palaeozoic rocks are no more seen until Mount 

 Bisehoff is reached, being completely covered by the basalt, except 

 in one instance where river action has cut through it. This being 

 a dark augitic rock, often very ferruginous, vesicular, and occasion- 

 ally amygdaloidal, its decomposition has furnished a rich chocolate- 

 coloured soil, which will account for the dense vegetation of this 

 part of the island. 



Mount BischofF has an altitude of 3,500 feet [Gould says 2,500 

 feet. W. B. C] above the sea, and but little more than 1,000 feet 

 above the surrounding basaltic table-land, the ascent being more or 

 less gradual from the coast. It consists of erupted euritic porphyry, 

 which forms a crescent-shaped ridge on the summit, the extremities 

 of which ridge are not more than a quarter of a mile apart, the 

 intervening space being a horseshoe depression or basin. It is 

 here that the richest deposits of tin ore exist. This basin looks 

 south, and presents a natural outlet to the surrounding table-land, 

 while the northern and western slopes are exceedingly steep, pre- 

 senting a mean angle of descent of 35°. The porphyry is the 

 chief matrix of the tin ore. It has burst through the transition 

 strata which repose on the slopes of the mountain, displacing, 

 contorting, and folding them in a most fantastic fashion. These 

 strata chiefly consist of clayslate, sandstone, and quartzose rock ; 

 the former being frequently highly charged with sulphides of iron 

 and tin. The tin ore occurs as a binoxide, and traverses the por- 

 phyry in veins and lodes ; the breaking up of which by subsequent 

 eruptive forces scattered the tin ore on the slopes of the mountain 

 in the form of talus. The transporting and arranging pow r er of 

 water as a cosmical agent seems to have played no part whatever 

 in these stanniferous deposits ; inasmuch as the particles and nug- 

 gets of the ore afford no evidence of having been subjected to 

 attrition, but, on the contrary, display sharp, irregular edges. The 

 disruptive force which shattered the lodes was probably the same 

 as that which affected the vast basaltic flow of the surrounding 

 country. The ore is not generally distributed over the sides of 

 the mountain, but exists in local patches of limited extent in the 

 majority of instances. In earnest of this I may mention that on 

 Messrs. Walker and Beecraft's section 240. tons of ore were taken 

 out of a chain square of wash-dirt, while at a distance of about 20 

 yards on either side of the cutting barely a trace of the ore could 

 be obtained. Some of the masses or nuggets of ore taken out of 



