Marshall. — Magnesian Bocks at Milford Sound. 483 



Hartzbergite. (Plate XLVIL, fig. 2.) 



Hand-specnnen pale-green rock with fine large bright 

 cleavage surfaces of enstatite, and between them granular 

 olivine. Scattered dark grains of iron-ore. 



Section. — Enstatite in large perfectly colourless transparent 

 plates showing good cleavage-traces. The plates include 

 poecilitic grains of olivine and iron-ore, and in some sections 

 a great deal of a carbonate showing rhombohedral cleavage. 

 As the analytical results give no calcium in tbe rock, this 

 carbonate must be magnesite. Its birefringence is similar to 

 that of calcite, giving even in thin sections colours and even 

 white of the higher order. The iron-ore is metallic in re- 

 flected light, and absolutely opaque even in the thinnest 

 section. No cleavage discernible. Attacked by HC1, so appa- 

 rently magnetite. The alteration of enstatite into magnesite 

 appears rather peculiar. All the rocks found as boulders 

 have evidently rolled down in stream-channels and landslips 

 from the hillsides that are thickly covered with forest. 

 The change may be due to the fact that percolating waters 

 are more than usually charged with organic acids derived 

 from the mass of decaying vegetable matter that forms the 

 surface soil. 



A partial chemical analysis was made of the hartzbergite, 

 with the following result : — 



Total ... 99-78 



A. Hartzbergite, Milford Sound; anal., P. Marshall. B. Hartzbergite, 

 Olivine Range, N.Z., Q.J.G.S., 1890. C. Hartzbergite, Oregon 

 County, U.S.A. 



In this analysis the high percentage of loss is evidently 

 due to the presence of magnesite. The iron-oxides and chro- 

 mium-oxides are at present only approximately estimated, for 

 ordinary methods of separating these substances fail. The 

 iron-ore has been separated by magnetic means, and the total 

 quantity of iron-oxides and chromium-oxides estimated. The 

 ore completely dissolves in HC1. This must indicate that 



