378 MR. CLOUGH AND DB. POLLARD ON SPINEL [Aug. 1899, 



minerals which we have stated to occur in the lumps. The spinel 

 has a conchoidal fracture and a vitreous lustre in the broken faces. 

 A few of the grains visible in the hand-specimen show small faces 

 of some crystal form, but Mr. Teall was unable to recognize any 

 face of a well-defined form, and the outline is often rather curved 

 or slightly mammillated. When the slice is viewed by reflected 

 light the spinel shows the same bluish colour as it does in the 

 hand-specimen ; but it is noticed that this colour is frequently less- 

 pronounced near the exterior of the grains than it is near their 

 interior, there being often a thin rim (from '02 to "05 mm. thick) 

 which contains streaks of different colours, some of them blue like- 

 the internal portions of the grains, and others of a brownish tint. 

 Some of the streaks are slightly curved, but have a general 

 direction nearly at right angles to the nearest side of the grain. 

 Others are straight, and are either parallel one to another or cut 

 one another obliquely, and they perhaps indicate a crystalline 

 structure. In the thin slice all parts of the grains are transparent. 

 The parts that seem bluest by reflected light have a faint brown 

 colour in transmitted natural light, while some of the marginal 

 streaks are nearly colourless. When rotated with crossed nicols 

 some of the blue parts allow a certain amount of light to pass ;, 

 they are not quite so black as some of the marginal streaks. Many 

 of the grains are crowded, particularly near their centres, with 

 small inclusions of calcite, phlogopite, and diopside, and perhaps- 

 nearly a quarter of the mass of some of the grains is composed of 

 these inclusions. The grains are also crossed by many irregular 

 cracks, some of which are filled with calcite, but we cannot discern' 

 any deformation. 



Separation and Analysis oe the Minerals. 



The rock was powdered in a steel mortar, passed through a sieve- 

 with meshes measuring about '22 mm., and the finest powder was 

 removed by sifting through cambric. Methylene iodide (sp. gr. 3*32) 

 was used to separate the minerals. 



The first fraction was fairly pure spinel, with some forsterite 

 and a few grains of iron, probably from the mortar. The latter 

 were removed by a magnet, and the sample was again separated,, 

 with the result that almost pure spinel was obtained (weight about 

 1-3 gramme). 



The methylene iodide was then diluted to about sp. gr. 3*29, when- 

 a second fraction of spinel came down, less pure than the first. This 

 was again separated, giving -4 gramme of fairly pure spinel, which 

 was used for prelimiuary examination. On further diluting the 

 methylene iodide to sp. gr. 3*2 the first forsterite fraction was 

 obtained. This contained a few grains of spinel, which were almost 

 entirely removed by a re-separation in methylene iodide of sp. gr. 3*3, 

 and about 2 grammes of practically pure forsterite were obtained. 

 A second fraction of forsterite was obtained on diluting to sp. gr. 3*15. 

 This was less pure, containing some yellowish grains. Its weight 

 was about -6 gramme, and it was used for preliminary examination.. 



