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



crossed nicols, and some rectangular sections, slightly elongated parallelograms, that 

 extinguish in directions parallel to the sides. It is not difficult in this case to recognise 

 nepheline, though in other cases it is disguised in the mass under the form of rounded 

 grains, but these are connected by a complete series of transitions with the distinct 

 sections just described. The granular shape prevents this mineral from being con- 

 founded with felspar, which never has this appearance except where, as in granitoidal 

 rocks, it is associated with quartz. With these microliths and grains of nepheline are 

 associated small prisms of augite, slightly yellowish or brownish and with vague out- 

 lines, some of which have large extinctions. Owing to the difference of refractive 

 index between nepheline and augite, the latter is sharply separated from its neigh- 

 bouring mineral. An interesting feature of this rock is the presence of large olivine 

 crystals, almost always fragmentary ; it is the only microporphyritic mineral present, 

 and is much larger than the two species just mentioned. The sections of olivine are 

 rugose, almost colourless ; they are bounded by the traces of the dome and of the 

 prismatic faces ; they extinguish parallel to the line of their length. Often they suggest 

 by their shape a well-developed crystal of olivine ; at other times we see that they are 

 only fragments of a single individual, which can be readily restored with the help of 

 the corresponding pieces to be found in neighbouring sections. It is apparent that 

 these olivine crystals, belonging to the very first phase of consolidation, have been 

 subjected to dislocation and to the corrosive action of the magma. They are broken at 

 the edges, and sometimes the ground-mass has penetrated the interior of the crystal. 

 All the olivine sections are altered to yellow on the sides ; when the sections are small 

 this zone of hydrated ferric oxide is so much developed that nothing may remain but a 

 small colourless area at the centre of the section. A tendency to assume a fibrous 

 structure is noticeable in the sections of this mineral. Several sections of olivine are 

 often seen grouped and joined together ; in polarised light these clusters sometimes 

 exhibit phenomena that vividly recall those observed in twins, but here the planes of 

 junction are too vaguely indicated to allow a positive statement ; nevertheless there are 

 such sections showing two individuals laid together and having a shape perfectly recon- 

 cilable with that of well-known twins of the rhombic system. Amongst the accessory 

 minerals we must note black mica, occurring in irregular scales strongly pleochroic, and 

 giving straight extinction ; this mica is often intimately associated with the decom- 

 posed olivine ; it sometimes even occurs as an inclusion in that mineral, which contains 

 also some particles of secondary calcite. This carbonate is present also in very fine 

 filaments in the ground-mass ; it is recognised by its irisation, by the twins parallel to 

 — tjr R, and by its cleavage. Worthy of interest is the presence of very numerous 

 minute grains of perowskite distributed throughout the ground-mass, where they play 

 a part almost as important as that of magnetite. Perowskite is hardly ever found in 

 well crystallised individuals, though sometimes traces of octohedra can be made out. 



