Vol. 55.] AND FOKSTEKITE FKOM THE GLENELG LIMESTONE. 377 



Besides the minerals already mentioned, and besides the f orsterite 

 and the spinel, we have also noticed in the limestone various scales 

 of graphite, in association with small quartzose lumps, and a rather 

 dark hornblende, either in scattered crj^stals in the calcareous 

 matrix of the rock, or aggregated together into rather large lumps 

 and containing many inclusions of a colourless pyroxene (7694). 



The thin slices of specimens obtained from rather more than ^ 

 ■mile north- north-west of Bailamhuilinn (7690), and from j mile 

 above the foot of Allt Easan Mhic Gorraidh (7917), show the small 

 serpentinous granules which are so abundant in the limestone. In 

 these slices all such granules are seen to consist either of serpentine 

 or of a partially serpentinized mineral, with high refraction and 

 strong double refraction, which closely resembles the olivine of many 

 -eruptive rocks. This mineral is also indistinguishable optically 

 from the olivine-mineral in specimen 7923, from rather more than 



1 mile east of Sgiath Bheinn ; the last-named has been analysed, and 

 £as thus been proved to belong to the variety called forsterite. 

 The olivine -serpentine granules in slice 7917 are often from 



2 to 3 mm. long. Within them are many parts which consist of 

 fresh olivine, and are quite colourless when viewed by transmitted 

 natural light, but these parts are separated one from another 

 by thin anastomosing veins of serpentine and of some black 

 granular ore, like those common in the olivines of other rocks. 

 'The black parts are sufficiently numerous to cause many of the 

 granules in the hand-specimen to appear almost black. The 

 granules are rounded or rather irregular in outline, and they show 

 oao well-defined crystalline faces or cleavage. Grains of diopside are 

 abundant in the same slice, and they sometimes form thin rims 

 round the olivine-serpentine granules. 



Spinel has not been noticed, except in the exposure rather more 

 than I mile east of Sgiath Bheinn.^ Here it occurs in grains, often 

 from 2 to 5 mm. in diameter, embedded in lumps 2 or 3 inches 

 long, which project slightly from the limestone and weather in 

 most parts with a brown crust. Besides the spinel, these lumps 

 'Contain also forsterite, serpentinous products of decomposition, 

 diopside, calcite, and a few flakes of phlogopite. The grains of 

 spinel are at once distinguished in the hand-specimen by their 

 almost opaque, pale blue colour. They sometimes occur in close 

 <jontiguity, and are so numerous that perhaps they form a fifth or a 

 sixth part of some of the lumps. The greater part of each lump is 

 evidently composed of forsterite or of the yellowish-green serpen- 

 tinous streaks which are closely associated with it, and give the 

 prevailing colour to the polished face of the specimen. The 

 forsterite-serpentine pieces are like those in specimens 7690 and 

 7917, except that they are rather larger and much more closely 

 aggregated together. They contain inclusions of all the other 



^ [Since this paper was read, spinels have been found in other localities. In 

 flome of these localities the mineral occurs in fairly well-developed octahedra, 

 apparently black, but really dark green. Spinels of a violet colour have also 

 been noticed in the locality near Sgiath Bheinn.— J^j^e 14th, 1899.] 



