BY W. H. TWELVETEEES AND W. F. PETTERD. 13- 



Garnet. — This is plentiful. When it occurs in such 

 rocks it is usually referred to the variety- of calcium-iron 

 garnet known . as melanite. It is in forms of the trapezo- 

 hedron and dodecahedron, and sometimes zoned. In thin 

 section the colour is yellowish to reddish brown. Its 

 sections are margined brown, and traversed by irregular 

 iron-marked fissures. It is quite common to find it inter- 

 grown with and enclosing crystals of hornblende and 

 augite. 



Hailyne (Nosean). — This mineral is abundant, and 

 gives sections approximating to faces of the cube (100), 

 dodecahedron (110), and trapezohedron (211). Hexagonal 

 sections are common ; no trigonal ones. Rounded grains 

 very frequent, and crystals with a mutilated and corroded 

 apjjearance. The mineral is sometimes colourless, but 

 generally, characterised by a soft slate-grey tint in the 

 peripheral parts, shading off towards the interior. The 

 colour is deepest at the border. Dark striae are often seen 

 proceeding from the faces inwards, sometimes in sets at 

 intersecting angles. There is none of the blue tint which 

 is seen in some haliynes. The interiors are full of granular 

 microlites ; nearlj" all are undergoing decomposition into 

 natrolite or other soda products represented by divergent 

 scaly aggregates. Crystals of haiiyne are often enclosed 

 in the large felspars. 



Nosean and haiiyne are classed together by Rosenbusch 

 under the group name haiiyne. They are both silicates of 

 alumina and soda, but in the haiij'ne there is the addition 

 of lime. Dana* gives the percentage composition of the 

 two minerals as under : — 



Sulphur 

 Silica, trioxide. Alumina. Lime. Snda. 



Haiiviie... 32 M-2 27-2 10-0 16-6 = 100 

 Nosean... 31-7 14-1 26-9 - 27-3 = 100 



Haiiyne often has a blue tint. Where this tint is absent 

 and no lime separates out during decomposition, it is 

 impossible to distinguish the two optically. These facts 

 have to be borne in mind when considering authors'' 

 references to either of these two minerals. 



HoriMende. — This is a somewhat peculiar variety. It 

 has the black colour of arfvedsonite to the eye,_ and is 

 deep green, sometimes nearly opaque in thin section. If 

 the section is at all thick it is opaque. The pleochroism is 

 strong. a = yellowish green ; ft = very dark green, 

 sometimes opaque ; f = very dark green, sometimes 

 opaque. This absorption scheme r ^ i) > a agrees with 



* System of Mineralogy, 18'J8, pp. 431-2. 



