444 Major G. W. Williams — 



felspar in a fine groundmass. It consists entirely of orthoclase and 

 iron oxide, the latter representing some ferromagnesian mineral. 

 The felspars of both generations are prismatic in form, and the 

 structure of the groundmass is typically orthophyric. This is the 

 dyke-facies of a syenitic rock. 



122 and 123. Porphyry, — A conspicuously porphyritic rock, with 

 phenocrysts of orthoclase in a fine groundmass of quartz, orthoclase, 

 and red iron oxide. The orthoclase is much decomposed, and the 

 original character of the ferromagnesian minerals is undeterminable. 

 The groundmass is microgranitic in structure. 



124. JEgirine-riebecJdte-granite. — A remarkably fresh rock, con- 

 sisting essentially of quartz, perthite, aegirine, and riebeckite. The 

 felspar is entirely microperthite of a finely intergrown type, in 

 idiomorphic crystals of tabular form. The quartz is interstitial 

 and the last mineral to crystallize. Ferromagnesian minerals are not 

 abundant ; chiefly emerald-green aegirine in idiomorphic prisms 

 with the characteristic slight pleochroism and small extinction angle ; 

 there are also a very few crystals of pale-brown or colourless augite. 

 Deep blue riebeckite is much less abundant than aegirine ; the larger 

 crystals are nearly opaque even in a very thin slice. The only 

 accessory seen is a little magnetite. 



125. Riebeckite- cegirine-granite. — This rock is closely related to 

 the last, the chief difl^erence being the predominance of riebeckite 

 over aegirine, and a slight tendency to micrographic structure. All 

 the minerals show precisely similar characters and further descrip- 

 tion is unnecessary. 



129. Biotite-augite- syenite. — A rock of fairly coarse texture and 

 even grain, consisting of felspar, biotite, augite, and riebeckite. The 

 felspar is chiefly perthite with a subordinate amount of plagioclase, 

 having a refractive index lower than Canada balsam (albite). The 

 biotite, which is very fresh, is of a deep chestnut brown colour, with 

 very strong pleochroism ; the augite is pale-green with slight 

 pleochroism and very strong birefringence ; therefore, probably 

 ferriferous. There is a little deep blue-green riebeckite in ragged 

 prisms, often intergrown with augite. The structure is hypidio- 

 morphic, with a tendency to parallel arrangement in the felspars ; 

 a typical syenite. 



130. Biotite- granulite. — An aggregate of quartz, perthite, and 

 red-brown biotite, showing a granulitic structure. Some large 

 crystals of felspar have a somewhat porphyritic tendency, but other- 

 wise the structure is even-grained and granular, of the aplitic type. 

 The most noticeable feature is the intense colour of the biotite 

 (lepidomelane), which is quite opaque in the position of maximum 

 absorption. 



131. Hybrid rock{1). — Large phenocrysts of orthoclase felspar, 

 somewhat rhombic in form and full of inclusions, in a ground- 

 mass of very variable texture, composed of quartz, perthite, ortho- 

 clase, and green hornblende. In parts the groundmass is granulitic 



