180 THE GEOLOGY OF THE CANOBOLAS MOUNTAINS, 



Composition : essential constituent, felspar; minor constituents? 

 segirine-augite, magnetite, sagenitic rutile, and secondary chalce- 

 dony. The large felspar-phenocrysts consist of microcline 

 microperthite and cryptoperthite, as in X.633. They are often 

 almond-shaped. The smaller phenocrysts are almost idiomorphic,. 

 and have the nephilinitoid habit, and the same composition. The 

 corroded almond shaped phenocrysts have a rim of clear anortho- 

 clase. The base consists of nephilinitoid and lath-shaped 

 microlites of anorthoclase. The next in order of abundance is a 

 brown, non-pleochroic mineral, with concretionary structure, and 

 a very low, yet noticeable, double refraction. This mineral is 

 probably chalcedony stained with limonite, and may represent 

 infilled miarolitic cavities. Limonite (secondary) occurs as 

 strands through the phenocrysts and base alike. Magnetite 

 occurs sparingly as round, corroded, phenocr3^sts. A few idio- 

 morphic grains of light green segirine-augite occur in the base. 

 Sagenitic rutile and apatite are often included in the felspar. A 

 little secondary chlorite and serpentine are also present. 



Name : a t3^pical Solvsbergite. This rock accurs also as the 

 plug of The Young Man. 



NoX.609. Porphyritic Trachyte. Loc: Bald Hill (?) 



Handspecimen : compact, porphyritic, flesh-coloured dyke-rock, 

 with an aphanitic base. 



Microscopic texture : holocrystalline ; porphyritic hiatal ; 

 with micro- to crypto-crystalline trachytic base. 



Composition : the essential constituent is felspar (over 90%). 

 This is of two generations. The phenocrysts have two distinct 

 cleavages, parallel to (010) and (001), and a parting parallel to 

 (100). Carlsbad twinning is common; Manebach twinning also 

 occurs; in addition faint albite-twinning is occasionally observed, 

 and, if not, there is marked shadowy extinction due to ultra- 

 microscopic twinning. The crystals are rounded, as in X.GiS, 

 and have a rim of clear orthoclase. The refractive index is less 

 than that of Canada balsam. The habit most commonly observed 

 is the prismatic with be forming an angle of 88*^. The faces 



