118 



Hi. Petrographiral Descriptions of the Various Types of In- 

 trusive Hock in the Houghton District. 



Note. — The slide numbers refer to the numbers in the 

 catalogue of the "Collection of Slides" of the Adelaide Uni- 

 versity. 



581. Locality. — In Torrens Gorge west of main intrusion 

 (Section 5521, Hundred of Yatala). Texture. — Hypidiomor- 

 phic granular grainsize, rather uneven, averaging about 2'5 

 mm. Minerals present in decreasing order of abundance. — 

 Plagioclase rather decomposed, and an oligoclase whose ex- 

 tinction angles in the zone perpendicular to (010) vary up 

 to 8°. Orthoclase, in amount almost equal to the plagio- 

 clase, in large or small rounded or irregular grains, often 

 twinned. It shows shadowy extinction. Quartz is included 

 in rounded grains, and around these and along the borders 

 of the felspar grains kaolinization has commenced and is pro- 

 ceeding inwards. It contains among the kaolin (or musco- 

 vite) small, scattered flakes of biotite, probably secondary. 

 Biotite is present in brown-green plates, bent and frayed, and 

 partially chloritized. Small patches occur of an aggregate of 

 quartz and biotite grains recrystallized under pressure. 

 Quartz is present in considerable amount, much strained, 

 though recrystallized crush areas show much less straining. 

 Some, from its association with secondary carbonates, appears 

 to have been introduced. Siderite is present in irregular 

 patches, with characteristic cleavage and occasionally outline, 

 negative uniaxial optical character, and weak pleochroism. 

 Its strong birefringence being noticeable without," the 

 analyzer can observe the difference in the relief of the sur- 

 face of a grain as it is rotated above the polarizer. Some 

 large ragged crystals of magnetite, probably primary, are 

 present, and smaller dusty particles in the bitotite and side- 

 rite are most probably secondary. A little apatite is present. 

 Narn e . — Epi-granite . 



613. Locality. — In the Torrens Gorge to the east of the 

 main intrusion. A section of a dark band of rock in the 

 gneissic mass. Texture. — Hypidiomorphic granular. The 

 predominant mineral is an acid plagioclase, with extinction 

 angles of 12° in sections perpendicular to (010) indicating that 

 it has a composition of Ab,2 An,. Almost equalling it in 

 amount is orthoclase in rounded untwinned grains, while 

 closely following it in the order of relative abundance are 

 pale-green faintly pleochroic uralite and flaky biotite, green 

 to brown. The determination of the felspars is rendered very 

 ■difficult by the presence of a fine network of secondary epi- 



