125 



Considerable quartz is present, but is apparently largely sec- 

 ondary, being aggregated in long bands and filling cracks in 

 felspar crystals. It often occurs in areas witb a slight lepi- 

 doblastic arrangement, associated with biotite. The scaly 

 arrangement is often perpendicular to the schistosity of the 

 rock. Biotite occurs in some amount, some primary, some 

 secondary. The latter occurs in the quartz aggregates be- 

 tween the felspar grains. A great deal of magnetite is present 

 in irregular grains and crystals. A little secondary musco- 

 "vite has been formed from the felspar. The rock may be 

 classed as an epi-syenite. 



The Recreation-ground schist is rather similar macro- 

 scopically, but under the microscope is seen to contain con- 

 siderable acid plagioclase and sericite bands, in which occur 

 also biotite and magnetite. A great deal of the smaller flakes 

 of biotite may be secondary, and considerable quartz is pre- 

 sent, which also is most probably secondary. This also is 

 an epi-syenite. 



548. Gneissic aplite. Locality. — From the quarry behind 

 the Houghton School, Section 5519, Hundred of Yatala. 

 M acrosco'pically white and cloudy, with parallel narrow bands 

 'of quartz and spotted with a little red haematite. Mio-oscopi- 

 cally (see plate iii., fig. 5) it is seen to be composed of long 

 parallel bands of quartz, considerably strained, among 

 rounded or irregularly-shaped grains of felspar, chiefly micro- 

 cline-microperthite, together with some orthoclase and a little 

 plagioclase, probably acid oligoclase. A single rod or plate 

 of ilmenite is also present, and some irregular grains. The 

 haematite has been secondarily introduced : it was not form.ed 

 from the grains' of iron -ore in the slide, as these are quite 

 fresh. A very little muscovite is present, apparently primary. 



This type of aplite is quite common in other places to 

 the west of the intrusion, and also occurs associated with the 

 rocks of the titaniferous magma at Yankalilla. 



"Yatalit-e," a new type of pegmatite. 



In general every type of plutonic rock may have asso- 

 ciated with it a pegmatite, which may be said to reflect in 

 •exaggerated form the distinguishing features of the parent 

 magma. Granite pegmatite, for instance, is composed of the 

 minerals most characteristic of granite quartz and acid 

 felspar ; it may contain also the ferromagnesian minerals typi- 

 cal of the granite, but usually it contains in increased amount 

 the apatite, fluorite, and rare earth minerals that are present 

 in but small amount in the main granite mass ; or, again, the 

 syenites of the gib rock near Mittagong, in New South Wales, 

 bave their distinguishing features accentuated in their pecu- 

 liar pegmatite, which has been described under the name of 



