TRANSACTIONS OF SECTION C. 135 



University Museum. Pebbles picked up iu the bed of the 

 Water of Leith, close north of Dunedin, are also of the same 

 character, and indicate the occurrence of the rock in situ 

 within the watershed of that stream. 



The rock is quite dense, nearly jaspery in aspect, and 

 breaks with smooth conchoidal fracture. Porphyritic inclu- 

 sions are small, and very sparingly distributed. Its colour 

 varies in shades of light and dark greyish-green, the lighter- 

 coloured varieties showing generally darker flecks and wavy 

 bands. The rock composing the pebbles found in the Water 

 of Leith is greenish-black, and somewhat resembles Lydian 

 stone. 



The appearance of thin sections of the rock, as seen under 

 the microscope, is very difficult to describe. Under a high 

 power there is seen a colourless transparent ground -mass, 

 which between crossed nicols, and on rotation of the stage, 

 proves to be allotriomorphic nepheline, possibly intermixed 

 with a slight percentage of glass and, sparingly, with lath- 

 shaped microlites of sanidine, though these occur in places 

 numerously and closely aggregated. This ground-mass is more 

 or less densely filled with pale-greenish microlites (ffigirine ?), 

 their uneven distribution producing light, dim, and nearly 

 opaque portions. They are aggregated in compact groups, 

 arborescent forms, narrower and wider, straight and wavy 

 strings ; and such strings also close to form figures of most 

 varied shapes, within which the ground-mass of allotriomor- 

 phic nepheline appears sometimes finer, sometimes coarser 

 granulated than the mass around (see Fig. 6). Occasionally 

 tufts and denser strings of these microlites enclose smaller 

 portions of the ground-mass, thereby producing a kind of 

 ocellar structure, as noticed by Eosenbusch in European 

 phonolites, in which, however, the enclosed part consists of 

 idiomorphic nepheline. Larger and smaller brown spots of 

 ferric hydrate, apparently originated through the decomposi- 

 tion of these microlites, are more or less abundant in parts of 

 every section. Solid grains of iron-ore are very scarce. Augite 

 is tolerably abundant in well-formed columnar crystals, from 

 about 1mm. to less than 0-02mm. in length, and of green, 

 occasionally greenish-yellow and light-purple, colour. Most 

 crystals have a broad black margin, composed of dusty iron- 

 ore, and such dusty ore also more or less invests the inside of 

 the crystals ; some, especially among the smaller crystals, 

 are rendered quite black-opaque by it. Idiomorphic nephelines 

 and sanidines are rather rare, irregularly-outlined grains of 

 these minerals more common. Needles of apatite seem also 

 rare. Particles of perfectly isotropic water-clear glass, con- 

 taining abundant gas-cavities usually arranged in lines, occur 

 rather frequently, and are sometimes of considerable size — 



