BY W. G. WOOLXOUGH. 539 



two such individuals are very nearly, but not quite, in parallel 

 position. The mineral is slightly decomposed, especially at the 

 periphery and along the spheroidal cracks, into a dark green 

 variety of serpentine which is noticeably doubly refracting, but 

 only very faintly pleochroic. 



It is doubtful whether any of the felspar is truly porphyritic 

 in the sense of Rosenbusch. Some of the sections are compara- 

 tively large, though always much smaller than those of augite or 

 olivine. All stages between these and the microlites of the base 

 can be traced, and no difference in composition is apparent. 

 Magnetite is abundant, the grains showing more or less crystal 

 form. 



The cavities in the rock are of two kinds — long, irregular 

 cracks, and round holes; The nature of the filling material is rather 

 puzzling. Some of it has all the characters of an acid felspar; it 

 is striated, and has a refractive index lower than that of Canada 

 balsam. In some cases the lines of felspathic material can 

 scarcely be called cavities, as they are only just discernible in the 

 groundmass. In these felspathic lines, but not in the round holes, 

 prisms of apatite occur abundantly. In both cases plates of 

 reddish pleochroic mica are very scarce. Some of the round 

 cavities contain undoubted zeolite, whose refractive index is very 

 much less than that of Canada balsam. It is colourless, with 

 opaque-white decomposition products thinly scattered through it. 

 It is more or less markedly in broad prismatic crystals whose 

 arrangement is either radial, irregular, or sheaf-like, suggesting 

 stilbite. Owing to the superposition of the crystals, it is difficult 

 to read extinction angles. Where this can be done the extinction 

 is nearly, but not quite, straight. The double refraction is of the 

 same order as that of felspar, though somewhat weaker. The 

 crystals are crossed by cracks at right angles to their length. 

 The elongation of the fibres is opticall}^ negative. All these 

 observations, so far as they go, are in favour of the zeolite being 

 stilbite. The broad distinct lines of colourless material through 

 the rock contain a good deal of this same zeolite which encloses 

 apatite; unless this apatite is secondary, the zeolite must be 



