178 THE GEOLOGY OF THE CANOBOLAS MOUNTAINS, 



No.X.GlS Biotite-Trachyte. Loc: a dyke, with glassy 

 selvage, between the Young and Old Man. 



Handspecimen : grey, slightl}- vesicular, with macroscopic 

 phenocrysts of biotite and felspar. 



Texture: porphyritic-hiatal; base pseudospherulitic as in X. 611. 



Composition : the phenocrysts of felspar have the properties of 

 sanidine; the microlites of the base have the same composition; 

 the former are fractured and corroded to a great extent. The 

 biotite is of a deep brov/n colour, and somewhat corroded. Small 

 flakes of it are included in the felspars. The aegirine-augite 

 extinguishes at 22*^. It is light green and weakly pleochroic 

 It occurs only sparingl}^, as fine idiomorphic phenocrysts, which 

 are tending to decompose into red iron-ores. Minute round 

 magnetite grains are also present in the base, as well as larger 

 cubical grains. 



Name : Pseudospherulitic Biotite-Trachyte. 



0,1 3. Grey Trachyte. Loc: Johnston's Pinnacle. 



Texture : holocrystalline, fine-grained, porphyritic in anortho- 

 clase; base trachytic in fabric. 



Composition : felspar forms about 90 % of the rock, and con- 

 sists of phenocrysts of microcline cryptoperthite, and of anortho- 

 clase; and a second generation of laths of the same mineral, 

 showing marked shadowy extinction. Next in abundance we 

 liave segirine-augite, showing strong pleochroism and almost 

 straight extinction. It occurs both as idiomorphic and allotrio- 

 morphic grains. The chief accessory is magnetite in corroded 

 grains of various sizes. Limonite is the most abundant decom- 

 position-product. 



Name : Porphyritic ^Egirine-Trachyte. 



0.28. Trachyte. Loc: summit of The Pinnacle. 



Texture : holocrystalline, porphyritic-hiatal, perpatic, with 

 even-grained trachytic base. 



Composition : the felspar, consisting of a few anorthoclase 

 phenocrysts together with the usual prismatic small laths of 

 anorthoclase, forms about 90 %. The rest consists of micropoi- 



