490 Transactions. — Geology. 



Aet. XLV. — Notes on a Hornblende Trachyte from 



Tawhetarangi. 



By W. A. MacLeod, B.Sc. 



[Read before the Auckland Institute, 10th October, 1898.] 



Plate XLVIII. 



A SAMPLE of this rock was left nie bv Mr. S. A. R. Mair, sur- 

 veyor ; and, on account of its peculiar nature and appearance, 

 I have, as a slight addition to the petrography of the Hauraki 

 Peninsula, written the following outline concerning it. 



My specimens were obtained at Tawhetarangi, or Amoteo 

 Bay, about ten miles north of Coromandel. Nearly two miles 

 inland, at an elevation of some 650 ft., Mr. Mair discovered 

 some boulders in the hillside debris along with others of a 

 coaly substance, and from appearances, and on account of 

 their being found near the top of the ridge, they could not 

 have travelled far, though he could obtain no trace of them in 

 situ, the country being covered with dense bush. 



A somewhat similar, though darker-coloured, rock forms a 

 dyke in Castle Rock, and quite lately Mr. J, M. Maclaren, 

 Dn-ector of the Coromandel School of Mines, discovered a like 

 rock forming a dyke in Moehau. 



The microscopic characteristics are very striking, espe- 

 cially the colour and arrangement of the constituent minerals. 

 The feldspathic portion is almost pure white, and crystalline 

 to semi- vitreous. In it are set the poi'phyritic hornblendes, 

 some of which are over -|-in. in length, whilst here and there 

 star-shaped twins of the same mineral stand out conspicuously. 

 At first sight the rock might be taken for one of the granitic to 

 syenitic type, but on closer investigation, by aid of the micro- 

 scope and chemical analysis, it is seen to be a trachyte. 

 Its fracture is uneven and massive, and its specific gravity, 

 which I determined both by the ordinary method of weighing 

 in air and water and also by the specific-gravity bottle, is 

 abnormally low, being 2-52 to 2-53. This is probably due 

 partly to infiltrated chalcedonic quartz and partly to the 

 numerous gas-pores contained in tbe feldspars. Chemical 

 analysis shows the following composition : — 



