287 



multiple twinning. Quartz is fairly abundant and often shows the outlines 

 due to corrosion, so characteristic of the rhyolites and quartz-felsites. The 

 brown mica occurs in hexagonal tables. It is very deeply coloured. Ex- 

 tremely thin flakes, examined in convergent light, show that the mineral is 

 biaxial with a wide but somewhat variable optic axial angle (sometimes over 

 73 in air). When examined in parallel light with the polarizer only a 

 marked dichroism is seen ; IB rich yellowish brown. 7 deep shellac brown. In 

 thin slices of the rock sections at right angles to the principal cleavage are 

 yellowish brown for rays vibrating at right angles to the cleavage (a, axis) 

 and opaque for rays vibrating parallel to the cleavage. The pleochroism in 

 extremely thin sections may therefore be defined as follows; a, pale 

 yellowish brown ; /3, rich yellowish brown ; 7, deep shellac brown. 

 Absorption a < ft < 7. In convergent light the dispersion of the optic 

 axes is very marked p < v. The biotite plates have been bent and 

 crumpled by movements in the magma and also more or less corroded and 

 frayed at the edges. The white mica occupies the same relation to the 

 ground-mass as the black mica. It appears to be an independent mineral. 

 It is colourless and devoid of pleochroism. Its optic axial angle in air is 

 about 56 . The occurrence of white mica (apparently muscovite) in a rock 

 of this character is exceptional. The ground-mass is mainly com- 

 posed of a somewhat granular isotropic substance deeply coloured in 

 places by stripes and patches of ferrite. The outlines of the ferrite 

 patches in many instances suggest the conclusion that they represent 

 corroded biotite. The rock shows a marked fluxion structure due to the 

 relation of the ferrite patches to the large porphyritic constituents. Other 

 specimens of the Lentrethen " porphyry " which have come under the 

 author's notice differ from the above in being more or less decomposed. 



Volcanic rocks of intermediate composition are extensively developed 

 in Glencoe and on the summit of Ben Nevis/ 1 ) The summit of Ben 

 Nevis is formed of a dark bluish grey rock in which small porphy- 

 ritic felspars are easily recognizable. Under the microscope this rock 

 is seen to be a very typical and only slightly altered hornblende- 

 andesite (see Fig. 1, Plate XXXVII). The plagioclase crystals have the 

 characteristic forms and inclusions. The hornblende occurs somewhat 

 sparingly; a, pale brown ; (3, deeper shade of brown ; 7, greenish brown. The 

 pleochroism is not strongly marked. The crystals often possess black 

 borders and sometimes the corrosive action has been so great as to leave 

 merely aggregates of magnetite which preserve more or less the forms of 

 the original crystals. The ground-mass is thickly strewn with minute dusty 

 particles. Examined with a high power the indistinct outlines of extremely 

 minute felspar rnicrolites may be often recognized. The microlites are so 

 closely crowded together that no interstitial glass is recognizable. This 

 rock is extremely interesting as it is at present the only good instance of a 

 hornblende-andesite known in the British Isles. It is almost certainly of 

 newer Palaeozoic age. 



(1) See Prof. JUDD, Q.J.G.S., Vol. XXX., p. 291. 



