ON THE ROCKS OF THE HAURAKI GOLD-FIELDS. 259 



alteration the felspars were not decomposed, the magnetite and 

 ilmenite were not oxidised and the ground-mass was little if at all 

 devitritied. 



The formation of pyrites was quite independent of this change 

 in the bisilicates, for many rocks with a chlorite contain no pyrites, 

 while in the older slates and lelsite tuft", pyrites is abundant without 

 any chlorite. Part of the pyrites appeared to have been derived 

 from magnetite or titaniferous magnetite, but this may not have 

 bpen the origin of it all. The formation of pyrites was not so 

 universal as the chloritization, it being absent, or in small quantity, 

 in many localities. 



Whether this impi'egnation of the rocks with sulphides took 

 place before or after the chloritization, or whether the two were 

 simultaneous, there is no evidence to shew. 



2)id Stage. — The second series of changes was the destruction of 

 the felspars and the formation of carbonates, kaolin, and a little 

 secondary quartz, most of the silica being eliminated. The chlorite 

 was l)Ieached and gradually removed, together with the apatite. 

 The ilmenite was changed into leucoxene and the ground-mass was 

 devitritied. 



3vd Stage. — In this the iron oxides were hydrated and gradually 

 removed, and the carbonates also were dissolved out ; leaving 

 notliing but quartz, kaolin, leucoxene, and pyrites. 



Tlie tirst series of changes took place at depths beyond the direct 

 action of surface agents, probably by means of warm acidulated 

 water and hydrogen sulphide. The second set of changes were no 

 doubt due to the direct action of cold carbonated 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 

 rock by denudation. The first series of changes is probably 

 connected with the volcanic action which caused the eruption of 

 the lava streams. 



The changes seem to be much the same as those that have taken 

 place in the Washoe District, Nevada, as described by Mr. G. F. 

 Becker, in the Reports of the U. S. Geological Survey for 1880-81. 

 But in the New Zealand rocks epiclote, if it occurs at all, is rare, 

 the chlorite is not stroiigly pleochroic, and the pyrites does not 

 appear to have been derived from the direct decomposition of the 

 bisilicates, altliough some of it may have originated from secondary 

 magnetite. Also the Washoe rocks appear to have no rhombic 

 pyroxene, but I have not seen Mr. Becker's full Monograph on 

 the district. 



STRUCTURE OF THE DISTRICT. 



If we omit the tuffs of Coromandel, Kennedy's Bay, itc, then 

 so far as my observations go, the only clastic rocks belonging to 



