456 C. E. Tilley — MetamorphisTn of 



cases been sufficient detrital silica to combine with all magnesia to 

 form forsterite. 



Dealing with these minerals we have : — • 



(i) Spinel. — -This mineral is characteristically of a light -green 

 tint, building little rounded grains or more rarely idioblastic. The 

 green coloration is to be ascribed to ferrous iron, indicating a member 

 of the pleonaste group. It may be present isolated in calcite or 

 dolomite, and also abutting or enclosed in original forsterite grains, 

 which now consist of forsterite residues surrounded by serpentine. 

 In some examples the spinel is bordered by a thin ring of serpentine, 

 presumably derived from it. Marbles containing abundant light- 

 green spinel are well developed in the north-eastern corner of 

 section 63, hundred of Hutchison. 



(ii) Phlogopite. — This is an important constituent of the marbles 

 of this area. It is colourless to a very light brown, and is often 

 much bent and contorted. Cleavage-flakes give an interference 

 figure normal to an acute bisectrix, but the optic axial angle is so 

 small that the opening of the hyperbolae is very slight. It shows 

 peripheral and intercleavage alteration to a chloritic product. 



(iii) Apatite. — Is developed in little rounded to sub-hexagonal 

 grains, showing uniaxial negative character. It is commonly 

 associated with rocks in which phlogopite is present. 



(iv) Chrysotile-Ashestos. — Occurs in irregular veinlets in yellow- 

 green serpentine in the gorge of the Mine Creek. The fibres are 

 always elongated at right angles to the length of the veins. 



Under the microscope the asbestos is seen to be present in 

 anastomosing veins, cutting across the mass of serpentine. The 

 refractive index is less than that of C. balsam. It is optically 

 positive, and the optic axial plane is parallel to the length of the 

 fibres. The elongation is positive and the extinction is straight. 



(v) Diopside. — Occurring in the same marble are nodular masses of 

 green diopside, associated with vein quartz, to which the formation 

 of diopside is due. Where the quartz vein has penetrated the 

 dolomite it is surrounded by a ring of the green silicate. The 

 pyroxene has not been definitely found in this locality other than 

 with this association, but in other places it occurs as a normal product 

 of metamorphism not involving addition of material. 



(vi) Forsterite. — The serpentinization of the olivine of these rocks 

 is accompanied by a separation of magnetite granules at the borders 

 of the pseudomorphs and betrays the original presence of a ferriferous 

 forsterite. There must still be considerable iron present in some of 

 the serpentine, judging by the strength of colour that is developed 

 (328). In other cases the forsterite is clear and unaltered (290). 



Haematite may take the place of magnetite bordering serpentine 

 pseudomorphs (313). 



In the absence of silica the rock has recrystallized without change. 

 No. 307, from an outcrop in section 804 treated with Lemberg's 

 solution, remained unstained with the exception of a very small 



