148 JOURNAL OF SCIENCE. 



changes is probably connected with the volcanic action which caused 

 the eruption of the lava streams. 



The pyrites found in the rocks was probably formed while the 

 first series of changes was going on, but would be quite independent 

 of them. Pyrites is never found as an original mineral in lava 

 streams, but is always formed subsequently from the magnetite by 

 the passage of sulphuretted hydrogen through the rock. 



The gold occurs in the veins in four different ways (1) in auri- 

 ferous pyrites, (2) scattered in small grains through massive quartz, 

 (3) in threads or scales between the points of quartz crystals in comby 

 veins, the quartz at the base of the crystals being often stained red, 

 and (4) in calcite, but only very rarely. It is never found enclosed 

 in a quartz crystal. The auriferous veins usually contain abundance 

 of pyrites, but other sulphides — stibnite, blende, arsenical-pyrites, and 

 copper-pyrites — are in small quantity only, and these have been 

 introduced subsequently to the gold. The carbonates of lime and iron 

 have also been introduced into the veins after the quartz. 



Now how far do these facts of decomposition of the rocks and of 

 precipitation in the veins tally with each other ? The first change in 

 the rocks was the conversion of the ferro-magnesian constituents into 

 chlorite. Now these minerals are anhydrous bisilicates of lime, 

 magnesia, and iron, with some alumina ; while chlorite is a hydrous 

 magnesian unisilicate with some alumina. Consequently in the 

 process of transforming augite into chlorite ; silica, lime, and some 

 iron must have been liberated; and we can easily conceive that 

 the lime, being soluble, was entirely removed, while the silica and the 

 iron might have been deposited in the fissures and the iron converted 

 into pyrites by sulphuretted hydrogen. And if the ferro-magnesian 

 minerals originally contained gold it might have been in part 

 removed and deposited with the pyrites. During the second series 

 of changes, which I have described, no more iron would be removed, 

 but the whole of the chlorite with the remaining gold would be 

 dissolved with the silica of the felspars and auriferous quartz would be 

 deposited in the veins. If the decomposition of the felspars took 

 longer than that of the chlorites, which is very probable, pure 

 crystallised quartz might subsequently be deposited on the auriferous 

 quartz. In the third series of changes the carbonates, which had 

 been formed during the second series of changes, would be dissolved 

 and part may have been deposited occasionally on the quartz. 



It will be thus seen that the two sets of facts tally very well, but 

 there is no apparent reason why the sulphides of antimony, zinc, 

 arsenic, and copper, should have been formed subsequently to the 

 pyrites. Absence of gold in the well crystallised quartz shews that 

 silica continued to be removed after all the gold had gone ; and we 

 might account for the fine threads and scales of gold between the points 

 of quartz crystals by supposing that during the second or third series 

 of changes, the auriferous pyrites in the veins was, in some places, 

 dissolved and that the gold was redepositecl, while the sulphur and 

 most of the iron was removed as sulphate of iron, nothing but red 

 stains being left behind. 



