34 THE VOYAGE OF H.M.S. CHALLENGER. 



relief; their dicroscopism is slightly marked, — the rhombic sections just mentioned 

 exhibiting variations of tint from slightly greyish to brownish yellow. The colours of 

 polarisation, without being conspicuously vivid, present a peculiar appearance which 

 may be termed irisation. 



Nosean is also one of the microporphyritic elements ; its sections are square- 

 shaped, hexagonal, or octagonal ; the reentrant angles indicate multiple groupings. 

 The individuals of this mineral are not homogeneous, the interior of the sections being- 

 riddled with inclusions often disposed in a network. The peripheral zone is not so 

 dark in tint as the nucleus, being often greyish, or inclining to clear blue, or coloured by 

 hydrated ferric oxide. Polarised light fails to reveal the strise characteristic of certain 

 noseans, — the sections are perfectly isotropic. 



Besides magnetite found round the crystals of hornblende, as mentioned above, that 

 mineral occurs also in grains, as an element of the first generation, as inclusions in 

 certain constituent minerals, — in augite, for instance. 



The ground-mass is characterised by fluidal structure. The constituent minerals are, 

 hornblende excepted, precisely those that have already been recognised in the form 

 of microporphyritic individuals. This ground-mass consists almost entirely of small 

 lamellar felspars, which extinguish very often in directions parallel to the long edges, 

 and exhibit, in most cases, the Carlsbad twinning. These small crystals of sanidine 

 are associated and often combined with some sections of the same tint and appearance, 

 which sections, however, are broader, better defined, and are never twinned ; the outlines 

 of these sections and their optical properties enable us to recognise them as nepheliae. 



The most conspicuous mineral in the ground-mass is a microlithic pyroxene ; it has 

 a green tint analogous to that of the large crystals of the same species that occur 

 in this rock. In general the outlines of these small augites are indistinct ; they are 

 rather of the shape of elongated grains than prismatic, and where they are found to 

 have a prismatic appearance, the outlines are indented. They are twinned according 

 to the usual law of augitic pyroxene ; they are pleochroic like the microporphyritic 

 augites, and in some instances one can make out in vertical sections extinctions that 

 exceed 30°. It seems very probable that titanite is represented in the ground-mass by 

 some very small crystals. It remains to refer to one more mineral, namely, apatite. 

 It always occurs in very small individuals, the sections being frequently parallel to the 

 vertical axis ; they show traces of the pyramid, and not merely the pinacoid as is 

 usually the case. They extinguish parallel to the line of their length; sometimes a 

 slight dichroism is noticed, the rays vibrating parallel to the vertical axis being bluish, 

 those which vibrate in a plane perpendicular to the greatest length almost colourless. 



Mr. Buchanan collected in the fissures which in two places scored St. Michael's 

 Mount from summit to base, a substance having the appearance and hardness of quartz. 



