86 GEOLOGY OF MT. FLINDERS AND FASSIFERN DISTRICTS, Q., 



ulfcramicroscopic twinning, chiefly the latter. The phenocrysts 

 frequently show an orthopinacoidal parthig, and a corrosion-rim 

 of heterogeneous composition. Two cleavages occur, and are, in 

 some crj'^stals or in parts of certain crystals, quite rectangular, in 

 others not rectangular, The latter portions frequently exhibit 

 distinct albite twinning, and have a higher refractive index than 

 the others. They consist of albite or a variety of anorthoclase 

 near albite. This kind of felspar frequently forms a rim round 

 the sanidine-like variety. The extinction of the sanidine-like 

 felspar is 11°. We have, therefore, three closely related felspars, 

 often mutually intergrown : (1) soda-orthoclase, (2) microcline- 

 microperthite (anorthoclase), and (3) albite. These three felspars 

 commenced to form at the same time, but the excess of soda in 

 the magma probably led the albite to continue to form after the 

 others finished. The felspar of the base has the refractive index 

 of albite. Generally speaking, the microlites of nephilinitoid 

 habit have the properties of anorthoclase, and those of prismatic 

 habit the characters of albite. Some of the phenocrysta show 

 incipient decomposition to kaolin and a clear isotropic material 

 like analcite. The other minerals observed in the slide are: 



(a) magnetite in idiomorphic grains, forming about 2 % to 3 %, 



(b) zircon in idiomorphic prisms (acicular), forming less than 1 %, 

 and (c) minute grains of a greenish pyroxene and of a yellowish 

 mineral which is probably wiihlerite or eucolite, and also a few 

 specks of deep blue pleochroic riebeckite. 



Name : Soda-Trachyte, near Orthophyre. 



F1.14. Handspecimen : porphyritic white trachyte occurring: 

 near F1.13, and forming an older flow. 



Texture: porphyritic-hiatal with cryptocrystalline base, partly 

 pseudospherulitic (strahlenkornig), in part pilotaxitic. 



Composition : the most abundant components are felspar and 

 isotropic or nearly isotropic base. In addition a little magnetite, 

 acmite and rutile are present. The felspar is of two generations. 

 The first generation, that of the phenocrysts, consists of a soda- 

 rich variety of orthoclase which sometimes includes lamellae of 



