Morgan. — The Igneous Rocks of the Waihi doJdfichJ . 26y 



replacing lime-soda feldspar. It cannot be doubted that such a change has 

 taken place to some extent in the rocks some distance from the veins, but 

 more especially in the wall-rocks. The chief agents in ordinary propyliza- 

 tion are without much doubt carbon-dioxide and water, and at Waihi a 

 considerable amount of potash appears to have been introduced when 

 propylization took place, so that a propylitic facies, which is possibly some- 

 what unusual,* was brought about. 



The appearance of the prop3'litic facies seems closely connected in some 

 way with ultimate silicification in the neighbourhood of planes of fracture — 

 that is to say, with vein-formation. The concentration of metallic minerals, 

 and more especially of gold and silver in the veins, gives a high economic 

 value to any successful attempt to explain the various phenomena attending 

 vein-formation and the localization of ore-shoots and bonanzas. It seems 

 certain that vein-formation in propylitic rocks is due to ascending solutions, 

 which also are presumably the carriers of metallic minerals, and yet in the 

 Hauraki Goldfield the influence of the country on the localization of values 

 is enormous. As the basis of a working hypothesis, it may be suggested 

 that, in effect, a double circulation was set up by some unknown factor or 

 factors ; whilst carbon-dioxide and potash in aqueous solution made their 

 wav hundreds of feet from the vein-fractures, other substances from the 

 country entered the veins, and probably caused or aided the precipitation 

 of gold, silver, and other metals. Osmosis and electro-chemical agencies 

 may well be supposed to have been concerned in the double circulation thus 

 invoked. It is wished, however, rather to emphasize the conception that 

 there was interaction between the whole body of propylitic rock and the 

 ascending vein-formiiig solutions during the period of propylization. In 

 other words, propylization and vein-formation were contemporaneous and 

 to a great extent interdependent. 



Problems to be solved at Waihi and elsewhere are : What are the 

 conditions necessary to propylization ? Does the propylitic alteration im- 

 mediately succeed the extrusion and solidification of dacitic or andesitic 

 lavas, or does it take place at a later date ? Is it a long-continued or a 

 rapid process % Again, does vein-formation accompany propylization as a 

 more or less necessary concomitant, or does it simply follow mainly because 

 rocks with the propylitic facies afford better facilities in some way for vein- 

 formation ? The economic geologist will be further interested to learn, if 

 possible, why quartz veins in propylitic rocks more often contain appreciable 

 quantities of gold and silver than quartz veins in other classes of rocks. 



The discovery of the laws regulating the distribution of ore-shoots and 

 bonanzas in the quartz veins of the Hauraki goldfields probably depends 

 in great measure upon the scientific investigation not only of the ore-bodies 

 themselves, but of the enclosing rocks. It has been said by many persons — 

 some with a sound practical knowledge of mining in the Hauraki goldfields — 

 that it is impossible to discover such laws ; but the progress made in economic 

 geology during the past twenty years gives hope that rules possessing real 

 value may yet be formulated. 



(2.) Stratified Tuff of the Grand Junction Shaft. 

 At a depth of about 620 ft. the Grand Junction A or No. 1 shaft passes 

 from andesite into fine-grained volcanic debris or tuff, which at 630 ft. to 



* A similar facies is developed at Karangahake, and possibly elsewhere in the Hau- 

 raki Peninsula. 



