Marshall. — Gabbro of the Dun Mountain. 



321 



eluded in the confused mass of feldstone, brouzite, anthophyllite, 

 &c, referred to on page 118 of that report. 



Hochstetter* refers to dykes of diallage rock in the dunite 

 of this region. 



A visit to the district in December, 1906, enabled me to 

 collect specimens, though in the many localities in which the 

 rock was found I was una.ble to do anything in the way of 

 determining its field relations, for I found it nowhere in situ, 

 though blocks were abundant on the north-east slopes of the 

 Dun Mountain and in the valleys of the Maitai Stream and 

 Roding River, especially where they issue from the magnesian 

 country. Microscopic examination showed that the so-called 

 saussurite was entirely isotropic, as previously mentioned by 

 Hutton. In all my specimens the ferro-magnesian mineral was 

 diallage. 



In the fresh specimens the white mineral was absolutely 

 clear and colourless in section, but in weathered specimens it 

 was somewhat cloudy. A specimen was obtained absolutely 

 pure for analysis, and it gave the following result : — 



Total . 



98-34 



99-96 



1. Grossularite-gabbro, Dun Mountain, Nelson, New Zea- 

 land. 



2. Grossularite, River Iset, Peru. 



From a comparison with the adjacent analysis of typical 

 grossularite it will be seen that the chemical composition of 

 the mineral shows clearly enough that it should be referred to 

 grossularite. The percentage of magnesia is high, though 

 when the nature of the adjoining dunite magma is considered 

 such a percentage is to be expected. 



The specific gravity of the mineral confirms this result. 

 From a specimen of absolute purity the following result was 

 obtained : G. 3-502. The average specific gravity for the 

 examples quoted (" Dana's System of Mineralogy ") is rather 

 over 3-5. The refractive index has not yet been measured, 

 though from the aspect of the surface in section the value 

 appears below that usual for grossularite. 



New Zealand," p. 475. 



11 — Traus. 



