ON THE BARRIER RANGES SILVER FIELD. 18£> 



which decomposition and replacements have taken place below. 

 These changes, depending mainly on the action of chlorides, 

 sulphates, carbonates, and free carbonic acid, derived from the 

 surface decay of organic matters, and their inter-actions between 

 each other and the earthy alkaline and metallic salts out of 

 contact with the atmosphere, must necessarily have been very 

 slow. The formation of large bodies of altered lead ore has 

 apparently necessitated the lead being first deposited as a sulphuret, 

 by sulphated waters. Its subsequent alteration by carbonic acid 

 in the presence of liberated sulphur and oxygen has determined 

 as conditions directed, its conversion into cerussite or angle- 

 site, the presence of molybdic and phosphoric acids giving rise to 

 wulfenite and pyromorphite. The course of such operations, 

 other conditions being equal, has naturally followed lines of freest 

 circulation, reaching in some cases to considerable depths, leaving 

 zones of unaltered ore at higher levels. Changes such as these 

 extending from the outer sides of ore masses inwards, have 

 formed massive coatings, between which and the unaltered 

 material are found void spaces due to a change of volume and a 

 probable loss of material by solution. The completion of this 

 process on large compact ore masses has resulted in large cavities 

 at the centre, around which are grouped the various metallic 

 combinations in most beautiful crystalline forms. 



In describing these larger ore formations as being mainly due 

 to zones of decomposition, in which the removed material has- 

 been principally replaced by metallic ores, the evident effects of 

 earth movements must not be overlooked, since they have, no 

 doubt, by re-opening previously filled channels, assisted largely in 

 the free circulation of opposing solutions. The effects of earth 

 movements become more apparent as we pass from the stronger 

 lode developments into the South, Junction, or North mining 

 properties, where in some instances the wall rocks are better 

 preserved, by the sliding of the foot-wall on the hanging wall. 

 Some of the bulges have been brought opposite each other form- 

 ing narrow places in the lode, while between the opposite cavities 

 large deposits of ore have accumulated, around which the wall 

 rocks have apparently suffered in proportion to the size of the 

 ore bodies formed. 



It now remains to notice fissure veins, dependent mainly on 

 earth movements for their ore deposits. For this purpose we 

 will take the Umberumberka vein, because its extensive under- 

 ground explorations offer special facilities for the study, while 

 the effect of earth movements have been here brought more 

 prominently under notice than elsewhere on the field, so far as 

 observed. This vein has been worked for a length of about two 

 miles, some parts being very rich, especially in silver. The more 



