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



eye, in the ground - mass, and with the lens some grains of olivine may be detected. 

 These minerals are enclosed in a dark-coloured matrix. The rock has a plane fracture. 

 The microscopic texture is microporphyritic, and felspar and augite are present as 

 large crystals or as microliths. The latter, grouped in the ground -mass, belong to 

 a second generation. Olivine often appears in rather well -formed crystals. The 

 felspathic sections exhibit the interesting peculiarity of being sometimes twinned 

 according to the Baveno law ; two individuals with plagioclastic stria? are joined at 

 right angles, and extinguish simultaneously. These hemitropic lamellae give sym- 

 metrical extinction at 17°. Hence the felspar may be classed as labradorite or 

 bytownite. The twin of pericline is rarely seen, and the crystals of plagioclase are 

 generally broken and corroded by the action of the magma. They preserve their 

 freshness only in certain parts of the section. They are usually covered with a 

 network of viridite, which also penetrates the larger constituents of the rock. Augite 

 appears as a rule in patches without regular outlines, and this mineral is even more 

 corroded and broken up than the felspar. Crystals of augite are often seen broken 

 into a number of fragments which are piled up one on the other, yet they 

 may readily be reconstructed, for the corresponding pieces bear the form of the 

 primitive octagon of sections perpendicular to the vertical axis. The cleavage and 

 optical properties leave no doubt as to the determination of this mineral. It is some- 

 times twinned according to the ordinary law, and its pleochroism is very slight. 

 One can hardly see any difference in the absorption of rays vibrating parallel to a and 

 to 7; both are green. The augitic sections are penetrated by the same greenish 

 substance which forms veins in the felspars, and they are also surrounded by a zone of 

 pyroxenic microliths similar to those of the ground-mass. 



The olivine is entirely altered, and only pseudomorphs of it by serpentine are to be 

 found, but these furnish exact models of the primitive crystals. The pseudomorph 

 polarises in blue tones ; this homogeneous tint is not that usual in this alteration 

 product of olivine. Its sections are traversed by threads of opaque black granules 

 arranged parallel to the cleavage. These dotted lines trace out blunt-angled squares. 

 In the interspaces of the crystal, which sometimes correspond to the cleavages, calcite 

 has crystallised, and from these it extends in somewhat thick veinules, which subdivide 

 into fine ramifications, penetrating the serpentinous matter. Minute patches of calcite 

 are also seen in the ground-mass. Magnetite occurs in rather large sections, but in 

 this case it is never bounded by crystallographic outlines, and like most of the minerals 

 composing this rock it shows traces of corrosion. 



The ground-mass, in which fluidal structure is distinctly marked, is made up, with 

 the exception of olivine, of the minerals which have just been described. Felspar and 

 augite assume the form of microliths, and viridite penetrates all the interstices between 

 them. 



