MINERALOGICaL notes : NO. IV. ANDERSON. 269 



in PI. xlix., fig. 4, the diiference is slightly exaggerated 

 to show that the faces are not quite in one plane. Well- 

 defined cracks parallel to the basal pinacoid traverse the 

 faces in the prism zone ; by observing the direction of 

 these cleavage cracks one can easily distinguish c from x, 

 even when these are to all appearance coplanar and physically 

 similar, and the dome y is not present. This crystal, which mea- 

 sures approximately 2 X 1"5 X 1 cm., forms one of a small group 

 of felspar and smoky quartz crystals with a crumbly pegmatitic 

 matrix. Seated in parallel position on, and partially embedded 

 in, the figured crystal are small crystals of albite. 



Manebach twins are not met with so frequently as Baveno and 

 Carlsbad types. One good but rather decomposed example, 

 accompanied by orthoclase in Baveno and Carlsbad twins, albite 

 and smoky quartz was observed ; in habit and development it is 

 essentially similar to the crystal figured in PI. xlviii., fig. 5, 

 which comes however from Bolivia. In the Oban crystal albite 

 is in parallel position with the two segments of the Manebach 

 twin ; hence the albite also must be twinned on the Manebach 

 law. 



On the whole the orthoclase ciystals of Oban suggest a com- 

 parison with those described from Four-la-Brouque, France.* 



Uralla. 

 (Plate xlix., fig. 5). 



In the Museum collection are a few specimens of orthoclase 

 from the Rocky River, Uralla, which are in general very similar 

 to the Oban mineral. This similarity is no doubt due to their 

 having been derived from a geologically equivalent pegmatite. 

 All the Rocky River felspars, Mr. Porter informs me, were ob- 

 tained in the alluvial gold wash, where they are accompanied by 

 ilmenite, zircon, quartz and jasper. It is worthy of note that 

 no tourmaline or cassiterite is found in the Rocky River wash, 

 in which it differs from the alluvial drifts in the neighbourhood 

 of Oban. 



A Carlsbad twin from the Rocky River is interesting as an ex- 

 ample of what is sometimes described as a left-handed twin, as 

 distinguished from the other figured crystals (PI. xlviii., figs. 4, 6 

 and PI. xlix., fig. 4) which are right-handed. This is a 

 comparatively small crystal, measuring about 1 X "8 X '5 



< Gonnard— Bull. Soc. Fr. Min. vi., 1883, p. 265 ; Ibid., viii., 1885, p. 307 ; 

 Ihid., si. 1888, p. 177. 



