124 Transactions. 



One section of a banded type shows dense somewhat axiolitic narrow 

 dark bands, between which are clearer bands largely of very finely 

 crystalline quartz. A few very small crystals of feldspar represent the 

 only phenocrysts. 



An Acid Dyke in the Andesitic Fragmentals. 



Before leaving the subject of the rhyolitic rocks, mention must be 

 made of a very striking dyke, apparently of rhyolite, forming a conspicuous 

 feature of the landscape on the northern wall of the Awana Valley. It 

 can be seen readily from the open country near the top of the ridge crossed 

 by the inland or " bush " track from Harataonga Bay to Whangapoua. 

 Seen from there at a distance of a little less than three-quarters of a mile 

 it appears a great vertical wall, probably at least 100 ft. high, built of 

 horizontal columns apparently of rhyolite, which outcrops near it to the 

 south, and piercing andesitic conglomerates clearlv visible in the adjacent 

 bluffs. (See Plate" XXV, fig. 1 .) 



Petrography of the Dyke Complex in the Basement Sediments. 



Having read Hutton's (1869) description of the Mine Bay area, the 

 writer had anticipated that his own visit would yield him petrographic 

 material of very great interest. He was not altogether disappointed, but 

 found that practically all the dykes he examined were greatly affected 

 by decomposition or alteration of some kind or another. This fact 

 militates against the exact deciphering of some of the varieties. 



The list of rock-types examined includes (1) pegmatite or granite- 

 granophyre, (2) quartz-porphyry, (3) quartz-porphyrite and quartz-ande- 

 site, (4) porphyrites and andesites. Hutton's original list includes diorite, 

 quartz-porphyry, and felstone. In a later paper (1889) he describes an 

 elvanite, or dyke-rock showing quartz phenocrysts in a felsitic ground- 

 mass. Apparently he did not examine the rocks microscopically, and his 

 identifications are therefore by no means sound. 



Very few indeed of the rOcks have escaped the prevailing alteration. 

 This is exhibited in the conversion of feldspars to kaolin, often with calcite 

 and quartz as additional products, and the hydration of biotite and ferro- 

 magnesian minerals generally to chlorite, sometimes with the addition of 

 calcite. The production, and often introduction, of calcite is most general, 

 and strings of it seam the dykes and adjacent sediments. Pyrite is a further 

 secondary mineral which is occasionally abundant. In several instances 

 metallization has proceeded along the walls of dykes, producing small 

 lodes in which the commonest minerals are chalcopyrite, sphalerite, galena, 

 and pyrite. It is possible that the general alteration of the dyke-rocks 

 is a result of the same processes as gave rise to the introduction of ore- 

 minerals, but it must be admitted that the wall-rock of the dykes appears 

 to give no evidence of any of the changes expectable on that hypothesis. 



1. Pegmatite or Granite-granophyre. 



This is a. curious rock forming an intrusion on the south-east wall 

 of the copper lode at Miner's Head, and surprisingly free from signs of 

 having been affected by the near passage of the solutions giving rise to the 

 lode. The minerals are equidimensional, and form a mosaic reminiscent 

 of that displayed by granulites Much of the rock shows graphic structure. 



