374 T. Barron — A British Rock containing Nq)heline, etc. 



a landed structure is seen ; this is accentuated by the segregation 

 of iron oxide along the bands. 



Under the microscope, the banded rock shows a trachytic structure ; 

 it is of a non-porphyritic character, only a few large sanidines heing 

 scattered through it. These show irregular outlines ; are often 

 corroded; and when examined between crossed nicols, break up 

 into a granular aggregate. This arrangement of the larger felspar 

 crystals is characteristic of this lava-flow. The sanidine crystals 

 also show the irregular series of cracks at right angles to their 

 direction of elongation, which is so characteristic of them in all 

 lavas. The base of the rock is made up of felspar microlites. a 

 bluish-green mineral, and a colourless mineral of low refractive 

 index and double refraction. 



The bluish-green mineral occurs in small pieces, often grouped 

 together, and moulding the felspars in the same way as augite does 

 in many of the basic rocks. Between crossed nicols these patches 

 extinguish together, showing that they are parts of the same crystal. 

 Many of the pieces show cleavage-lines parallel to their sides, and 

 one or two likewise show the hornblende cleavages. The extinction 

 angle, measured from the longitudinal cleavage-lines, was 5°. 

 When rotated over the polarizer this mineral showed intense 

 pleochroism. Owing to the small size of the pieces I was unable 

 to obtain a satisfactory 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. 



„ „ b ... ... deep bine-green. 



„ ,, a ... ... pale yellow-green. 



All the characters above cited point to the conclusion that the 

 mineral is the soda-amphibole — riebeckite. But, as Bosenbusch 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>6>r. 

 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-porpbyritic patches of sanidine and 

 colourless diopside were observed; a few altered porphyritic 

 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 minei-al 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 



