258 Transactions. 



Art. XXXI. — The Igneous Rocks of the Waihi Goldfield. 

 By P. G. Morgan, M.A. 



[Bead before the Wellington Philosophical Society, 5th October, 1910.] 

 Contents. 



Introduction. 



Outline of geology. 



Previous jietrographic descriptions. 



Special petrography — continued. 



(2.) Stratified tuff of the Grand Junc- 

 tion shaft. 



Special petrography — I (,3.) Andesites and tuffs of the " Second 



(1.) Rocks of the auriferous series. [ Period." 



(a.) Quartz-andesites and dacites. (a.) Andesites. 



(b.) Wall-rocks. j (6.) Tuffs. 



(c.) Vein- material. (4.) Rhyolites. 



(d.) Discussion of nomenclature. 1 (a.) Spherulitic rh^'olites. 



(e.) Alteration of vein-bearing i (b.) Wilsonite. 



dacites. (c.) Tridymite-rhyolites. 



(/.) Significance of propylitic i (5.) Andesitic rocks of doubtful age. 



facies. ' Literature. 



Introduction. 



In leisure time during the years 1903-5 the writer was engaged in making 

 a petrographical study of the rocks occurring in or near the Waihi Gold- 

 field. The main object of this study was to obtain data that would be 

 of value in a detailed investigation of the geology and the mineral veins of 

 that district. It was thought that such an investigation would assist in 

 defining the limits of the auriferous rocks, and in discovering the mode of 

 origin of the veins and the source of their metalliferous constituents. More 

 especially it was hoped that clues to the laws regulating the distribution 

 of values in the auriferous veins and the depth to which payable ore might 

 be expected to persist would be foimd. 



In this work the writer was interrupted by a change of residence and 

 occupation. So far as his researches went, the possibility of data of high 

 economic value being obtained by close petrographical study, by chemical 

 analysis, and by other methods of scientific investigation was clearly in- 

 dicated. Since the study as originally planned by the writer remains 

 woefully incomplete, it is not possible to set forth all that might be accom- 

 plished by scientific work, nor is it advisable to state various conclusions 

 that are not fairly well supported by the evidence actually obtained. 



The object of the present paper is to put on record the main results of 

 the writer's petrographical work, with such descriptions as may be useful 

 to future workers. It is desired especially to direct attention to the nature 

 of the rocks enclosing the veins of the Waihi Mine, and to the type of altera- 

 tion that these rocks have undergone. 



Outline of Geology. 



The oldest rocks exposed in the neighbourhood of Waihi are the altered 

 lavas in which the gold-bearing lodes of the Waihi, Grand Junction, 

 and other mines occur. These rocks are very generally considered to be 

 quartz-bearing andesites and dacites, although a few years ago, when various 

 samples from the Waihi mines were determined by Professor W. J. Sollas 

 as altered pyroxene-rhyolites, some doubt as to their true nature arose. 



