SOURCE OF THE GOLD AT THE THAMES 147 



The gold appears to liave come out of the volcanic rocks and not 

 to have been introduced from below through lodes traversing the old 

 sedimentary rocks. The reasons for this opinion are (1) that after 

 nearly forty years' prospecting auriferous reefs have only beeu found 

 in the volcanic series or in the slates immediately in contact with them. 

 (2) The gold veins are often small, irregular and branching, some- 

 times only a quarter-of-an-inch thick, and often die out. They very 

 rarely lead into large reefs, and when they do so, these large reefs are 

 barren. (3) The amount of gold in the veins varies with the state of 

 decomposition of the country rock, the veins in decomposed rock 

 being richer than those in undecomposed rock. This being so it will 

 be interesting to see what process of decomposition has gone on in 

 the rocks, which has resulted in concentrating gold in the veins. 



The volcanic rocks themselves were originally lava streams of 

 that variety called andesite. They consisted of a ground-mass, partly 

 glassy and partly stony, containing abundance of fine grains of mag- 

 netite and crystals of titaniferous iron-ore. In this ground-mass were 

 larger crystals of lime and soda felspars and of some ferro-magnesian 

 minerals, usually augite alone, but sometimes with hypersthene in 

 addition, and more rarely hornblende. When fresh the rocks are 

 dark grey to black in colour, and a close inspection shows the small 

 white crystals of felspar embedded in the dark ground-mass. Small 

 patches of these undecomposed rocks are still found here and there, 

 But the mass of the rocks are now soft and light coloured, grey or 

 greenish, or occasionally red. The original dark colour of the rocks 

 was due to the iron ores and ferro-magnesian minerals they contained, 

 and the change of colour is due to the decomposition of these 

 minerals. In some places the magnetite has been changed into 

 hematite and the rock has become red, as between the Karaka and 

 Hape creeks, but this change is comparatively rare. In nearly all 

 cases the ferro-magnesian minerals — Augite, Hypersthene, and Horn- 

 blende, — have been altered into chlorite, and this newly formed 

 chlorite was also often deposited as infiltrations in the ground-mass, 

 giving the rocks a green colour. In some rocks the decomposition 

 has gone no further, but in others another change took place the 

 felspars being decomposed into quartz, calcite, and kaolin, while the 

 chlorite was gradually dissolved out, leaving the rocks nearly white, 

 but coloured grey by small specks of iron ore. At the same time the 

 titaniferous iron ore was changed into an opaque white mineral called 

 leucosene, giving the rock a spotted appearance. In some cases a 

 still further change took place, the iron oxides being hydrated and 

 gradually removed, and the calcite leached out, leaving nothing but 

 quartz, kaolin, leucoxene, and pyrites. 



-The first series of changes took place at depths sufficiently great 

 to be beyond the direct action of surface agents, and was probably 

 produced by the percolation of warm acidulated water. The second 

 set of changes were no doubt due to the direct action of cold carbo- 

 nated surface water in limited quantity ; and the third set of changes 

 to the same agent but in much larger quantity. The second and 

 third set of changes would be gradually brought about by the 

 removal of the overlying rocks by denudation. The first series of 



