132 KEPOPvT— 1891. 



conchoidal fracture or imperfect cleavage, and are greeu- 

 trauslucent on thin edges. Considering the purely macroscopic 

 aspect of the rock, I think the fittest term for it would be 

 " sanidine-nepheline-porphyry." 



The real ground-mass of the dark base of the rock is holo- 

 crystalline, and, with the exception of scattered, slightly 

 opaque patches, nearly colourless -transparent. Between 

 crossed nicols it is seen to consist of allotriomorphic nepheline, 

 intermixed more or less abundantly with lath-shaped raicro- 

 lites of sanidine. It appears, as it were, broken up into a mass 

 of granules with ill-defined outlines, polarising, on rotation of 

 the stage of the microscope, from bright colourless through 

 light to dark greyish-blue, whilst the feldspar microlites between 

 pass during this operation generally from colourless through 

 yellow to dark alternately. The forms of the granules are 

 quite irregular and undefined, though sometimes they appear 

 to have rudely hexagonal, square, or rectangular outlines. 

 Throughout this ground-mass there are now distributed as 

 porphyritic inclusions sanidine, nepheline, augite, hornblende, 

 olivine, iron-ore, apatite, titanite('?), and particles of clear glass ; 

 the special characters of each of which are as follow : — 



Sanidine. — Colourless, transparent ; sometimes in grains of 

 irregular outline, but generally idiomorphic, appearing fre- 

 quently in Carlsbad twins. Larger crystals are usually much 

 cracked and filled in the centre with dusty black matter, occa- 

 sionally also in lines parallel to the sides. The polarisation 

 colours are mostly shades of yellow, rarely deep-orange, light- 

 red, and greyish-blue. Occurs abundantly. 



Neplieline. — This appears but rarely in idiomorphic inclu- 

 sions; smaller than 0-5mm.|(7?, Figs. 2 and 5, PI V.). It is colour- 

 less-transparent, and small crystals are generally very clear 

 and little fractured {n, Fig. 2) ; only occasionally one dimmed by 

 fine dusty matter is observed {n, Fig. 5). In moving sections 

 in reflected light in different directions some crystals show 

 a bright bluish opalescence, similar to that of moonstone. 

 Larger crystals, projecting into the dark base, generally have 

 dusty matter and microlitic inclusions zonally arranged parallel 

 to their sides ; and a pale-cream-coloured, slightly opaque, and 

 finely-fibrous zone close along the sides indicates doubtless 

 commencing decomposition. Eoundish particles of clear glass 

 were also observed in some larger crystals. 



Augite. — This is abundantly distributed throughout every 

 thin section in well-developed columnar crystals of fine grass- 

 green colour, the crystals varying in size from over 4mm. long 

 and 2mm. broad to less than 0-02mna. by O'Olmm. ; and there 

 occur in profusion still smaller light-greenish and brownish 

 microlites of lath and other shapes, which may possibly belong 

 to the alkaline-bearing varieties of augite — " segirine " and " ac- 



