127 



iv. Bocks Related to those of the Houghton Intrusion 

 occurring near Aldgate. 



Near Aldgate Railway Station there is an occurrence of 

 granite rocks in a small creek, as recorded by Mr. Howchin.'^i^) 

 A couple of hundred yards up the hill to the east of this 

 there is an outcrop of rather fine-grained rock 0'3 mm., 

 rather dax'k in colour, which microscopically has a hypidio- 

 morphic to allotriomorphic granular texture, and very even 

 grainsize. It is composed predominantly of orthoclase in 

 very irregular grains, including a large amount of quartz 

 in small clear grains ; a considerable, though smaller, amount 

 of plagioclase is also present, an acid oligoclase in composi- 

 tion. The grains of plagioclase are in general less irregular 

 in shape than the orthoclase. Smaller, prismatic or oval, 

 green diopside is present. It is hardly noticeably pleochroic, 

 and its high extinction angles prove that it is not uralitized. 

 Nevertheless, it is completely surrounded by a fine dusty mass 

 of epidote. Quartz is present in some amount in clear, 

 rounded, or irregular grains. It appears to have formed unusu- 

 ally early, often crystallizing before the felspar. Irregular 

 to idiomorphic titaniferous magnetite is present in rather 

 large grains. There is no sign of gneissic banding. JVam^e. — 

 Diopside quartz syenite. 



The rock thrown out on to the mullock heap of the old 

 mine a hundred yards north of here i& of rather peculiar 

 nature. In hand specimen it is greyish-green, rather schist- 

 like, and obviously much sericitized, possibly due to the action 

 of ore-forming solutions. Microscopically (slide 607) it is 

 seen to consist of felspar and magnetite, with but little bio- 

 tite, the only ferromagnesian silicate present. The felspar 

 is acid oligoclase' and orthoclase in approximately equal pro- 

 portions. Sericite runs through the slide in strings and 

 wisps. Quartz occurs in small amount and interstitially. 

 Name. — Syenite, inclining to aplite. 



Another type of rock outcrops on the bend of the Aid- 

 gate to Adelaide main road by the Pound (Reserve No. 2, 

 Hundred of Noarlunga), by the granitic outcrop before men- 

 tioned. It is a fine-grained grey rock with brownish streaks 

 often running through it. Microscopically (No. 602) it con- 

 sists of felspar, uralite, magnetite, quartz, with accessories. 

 The predominant felspar is orthoclase, occurring in large to 

 small untwinned crystals, sometimes full of inclusions of 

 irregular grains of other minerals ; the orthoclase appears to 

 have been the last mineral to crystallize, and large areas of 

 it occur in optical continuity, in which the grains of the 



a9)0f>. cit., p. 250. 



