BY H. I. JENSEN. 91 



polarisation-colour is raised; hence a = c'; this was also found to 

 be the acute bisectrix, hence the mineral is optically negative. 

 The green pyroxene is undoubtedly cegirine. In the phenocrysts 

 it is surrounded by a rim of yellowish or brownish nonpleochroic 

 acmite with rather similar optical properties. The double refrac- 

 tion of the gegirine is about 0*050. Incipient decomposition to a 

 brownish serpentine and to ferrite, with the occasional appear- 

 ance of hornblendic cleavage, was noticed in the acmite. 



Colourless, isotropic areas were noticed. Their refractive index 

 was very low, and they probably consist of nosean decomposing 

 to analcite and zeolites. Some nosean aggregates have been 

 completely replaced by zeolites (natrolite and thomsonite). 



Magnetite occurs in idiomorphic grains. Apatite occurs in 

 stout acicular prisms interpenetrating both the pyroxene and 

 felspar. Fluorite occurs in irregular grains, or aggregates of 

 grains, known by their low refractive index, faint yellow or violet 

 colour, and their isotropism. There are also a few grains of a 

 bright yellow allotriomorphic mineral with high refractive index 

 and low double refraction, probably eucolite. 



Leaving out of consideration minor constituents, felspar forms 

 practically two-thirds, and pyroxene one-third, of the total bulk 

 of the rock. 



Name ; originally this was a nosean-bearing segirine-anortho- 

 clase-trachyte. 



Chemical Composition : 

 Per cent. Mol. 



SiO.^ 60-58 



Norm; 



Quartz I'iiO 



FeO .: 1-38 0019 Orthoclase 40*59 



MnO 0-04) , Albite 42-44 



NiO.CoO... 0-07/ ^^^^ Anorthite 6-39 



MgO 0-23 0-006 Diopside. .; 1-68 



CaO 1-74 0-030 Magnetite 2-09 



K.^0 6-87 0-073 Ilmenite 1*52 



Na.^0 5-01 0-081 Hfematite 1-50 



H^OlatlOOoC) 0-99( ^.,,^p^ Water 1-89 



H^Olabove 100«C) .. 0-90J 



TiO.2 0-83 0-010 Sum 99'70 



Sum 



