MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 173 



constituent of microperthite. In one specimen fairly large individuals without any 

 very definite crystallographic boundaries were observed in a fine-grained groundmass of 

 biotite and orthoclase. 



Next to orthoclase, melanite is the most important constituent of these singular rocks. 

 It is black, and possesses, when broken, a somewhat resinous lustre. Good crystalline 

 form is absent, as a rule, but perfect little crystals may occasionally be observed. The 

 dominant form is the rhombic dodecahedron {HO}. The edges of this form are 

 sometimes truncated by those of the icosi-tetrahedron {211}, exactly as is the case in the 

 well-known melanite from Frascati. The mineral fuses in the flame of the blow-pipe 

 to a black glass which is slightly magnetic. In thin sections the colour of the 

 melanite is seen to vary from a pale to a deep brown (see fig. 1, PL XXXVII.). The central 

 parts of an individual are sometimes more deeply coloured than the marginal parts, 

 and sometimes the reverse relation may be observed. The borders of the differently 

 tinted portions may correspond to the crystallographic outline of the individual, thus 

 producing true zonal structure, or they may be irregular. 



The individuals vary in size from very small grains, only "05 mm. in diameter, to 

 large crystals or irregular masses measuring 1 or 2 mm. across. Melanite is both 

 idiomorphic and allotriomorphic with respect to felspar. Iron-ores, sphene and biotite 

 occur as inclusions. 



The biotite appears black when viewed microscopically. Cleavage flakes, examined 

 with the microscope, appear a dull dark green by transmitted light, and are nearly 

 uniaxial. Thin sections at right angles to the principal cleavage change from dark green 

 to yellowish brown as the stage is rotated over the polariser. The individuals vary 

 considerably in size and are generally irregular in form. The larger flakes are often 

 corrugated. Pyroxene, iron-ores, garnet and occasionally felspar, occur as inclusions. 



The pyroxene is green both by reflected and transmitted light. It is quite 

 subordinate in quantity, as a rule, to the orthoclase and melanite. In one specimen from 

 the north-west slopes of Cnoc-na-Sr5ine and in another from the burn close to the marble 

 at Ledbeg it occurs abundantly, and makes with felspar the bulk of the rock. Melanite 

 is absent from these specimens. As a rule, the mineral is without any very definite 

 crystalline form, but sometimes the individuals are elongated in the direction of the 

 vertical axis and more or less idiomorphic in the prismatic zone. The forms recognisable 

 are {110}, {010} and {100}. The prismatic faces {110} are not uniformly developed in 

 the different crystals ; sometimes they appear only as slight truncations and sometimes they 

 are developed almost to the exclusion of the clino-pinacoid. The ortho-pinacoid is always 

 conspicuous when any trace of form is present. As already stated, the mineral appears 

 green in thin sections, but the tint is not uniform — the marginal portions being often 

 more deeply coloured than the central parts. There is also a faint pleochroism. The 

 least axis of elasticity makes an angle of about 40° with the vertical axis of the crystal. 

 All the above characters agree with those of pyroxenes known to occur in nepheline- 

 bearing rocks. Magnetite and apatite are present as inclusions. 



