108 



THE MINERALS OF TASMANIA. 



bonate, and the latter mainly in the form of minute par- 

 ticles and crystals of chloro-bromide or analagous minerals. 

 Sometimes it happens that the agencies of oxidisation affect 

 the lodemass to considerable depths, as has occurred at the 

 Magnet Mine, where the gossan has been mined at a depth 

 of over 500 feet, but the decomposed ore is in diminishing 

 length as level succeeds level. This is probably the greatest 

 depth to which the gossan is known to occur in this State ; 

 but in the well-known Dalcoath Mine, Cornwall, England, 

 the oxidised products have been found to a depth of 197 

 fathoms (Collins, Journal R. Inst. Cornwall, Vol. IX., 

 p. 471). 



Regarding the associated secondary minerals, it has been 

 shown that metallic sulphides exposed to the influence of 

 water free from carbonates are usually converted to sul- 

 phates, but the presence of carbonates determines the 

 formation of oxides, hydrates, and carbonates, the chromic, 

 vanadic, and other acids obtained from the minerals of the 

 adjacent rocks or the lode constituents also affecting those 

 which are formed in the material resulting from the altera- 

 tion of the original lode and contents. The gossan gener- 

 ally bears a relation to the normal ore-body, but it does 

 not always follow that a rich outcrop will cover a corre- 

 spondingly rich material below the zone of oxidation. The 

 gossan outcrop of silver-lead or copper lodes is of great 

 interest to the mineralogist, as it is from this that he 

 secures many of the most beautiful minerals occurring in 

 nature, both as regards attractive colouration and crystal- 

 lisation. It is in this laboratory that the oxysalts have 

 their origin, and form numerous homogeneous chemical 

 combinations known as mineral species. The beautiful 

 chromates, phosphates, arsenates, carbonates, and other 

 secondary minerals are almost confined to the zone of 

 oxidation, and would be practically unknown but for the 

 influence of this. Nature's alchemist. 



Limonite has the property in a high degree of agglutin- 

 ating sand and gravel into concretionary forms, which vary 

 in size from quite minute to masses a foot or more in 

 diameter. Although the shapes assumed are generally 

 very irregular and capricious, they are often more or less 

 rounded, occasionally tubular or box-like. They may show 

 a hollow interior surrounded by a comparatively thin crust, 

 enclosing mud, sand, or a loose stony kernel, in which latter 

 case they may distinctly rattle on being shaken. Such 

 examples are common about Ravenswood and numerous 

 other localities where decomposed effusive rqcks have been 



