96 A NEW BRITISH ROCK. BY T. BARRON 



figure from which to determine the axes of elasticity ; but 

 referring the colours to the crystallographic axes, the scheme of 

 pleochroism is as follows: — 



Parallel to c - - deep blue. 



,, M * - - deep blue-green. 



,,,,»- - pale yellow-green. 



All the characters above cited point to the conclusion that 

 the mineral is the soda-amphibole — riebeckite. But, as Eosen- 

 busch has shown, it is the axis a, and not c, which makes an 

 angle of 5° with the vertical axis ; the scheme of pleochroism 

 thus becomes a > B > c. This mineral forms fine ophitic 

 patches with the felspar, but in certain parts of the rock it 

 is replaced by a pseudomorph of iron oxide, which preserves 

 intact the outlines of the original patch. 



In one or two places glomero-porphyritic patches of sanidine 

 and colourless diopside were observed ; a few altered porphy- 

 ritic augites and a little altered aegirine were also present in 

 the section. 



Scattered through the base of the rock are some brownish 

 altered patches, which, when examined with a quarter-inch 

 objective, are seen to consist of felspar microlites, and another 

 mineral of lower refractive index, which remains almost 

 isotropic when rotated between crossed nicols. This mineral 

 occurs in irregular patches which extinguish in definite areas, 

 and mould the felspar microlites. In certain places four-sided 

 and irregular six-sided plates were observed, the former 

 extinguishing parallel to their edges, and the latter giving a 

 uniaxial, partial interference-figure in convergent polarized 

 light. Nearly all the plates of this mineral were slightly 

 altered, and some were in an advanced stage of zeolitization. 

 In extremely thin sections the difference in the polarization- 

 colours of this mineral and the felspar microlites was much 

 more plainly seen. From these optical characters I have 

 concluded that the mineral is nepheline. As a confirmatory 

 test, another thin section was prepared, and exposed to the 

 vapour of strong hydrochloric acid for half-an-hour. It was 

 then washed with warm water, covered with a saturated 

 solution of malachite-green, and set aside for fifteen minutes. 

 (This material was chosen in preference to fuchsine, because, 



