BY W. F. I»ETTEKD 69 



Oligoclase, with its narrow twin lamellae, is the 

 plagioclastic fel&par of our granites. Andesine 

 occurs in essexite at Port Cygnet. 



Microcline^ though chemically identical with ortho 

 clase, is triclinic in crystallisation. Basal sec- 

 tions microscopically shew a characteristic cross- 

 hatched twinning, due to the intersection nearly 

 at right angles of the twin lamellae of two types 

 (albiteand pericline). Seen in granite porphyry 

 at St. Marys, and in granite elsewhere. 



17 Galenite (Sulphide of lead.) 



Ax the Magnet Silver Mine somewhat fine pseudo- 

 morphs of this mineral, after sphalerite^, have occa- 

 sionally occurred. They are usually in irregular 

 groupings, with drusy surface and glimmering 

 lustre. 



18 GoTHTTE (Hydrous sc'^quio.ride of iron.) 



Occurs sparingly, usually as a coating, at the last 

 named locality. 



19 HisiNGERiTE (Hydrated ferric silicate.) 



In amorphous masses of an intensely black colour, 



with a conchoidal fracture. 

 In lode matter exposed in the lower tunnel of the Com-- 



stock Mine, Comstock District. (Mr. G. Waller, 



loc. cit.) 



20 Hornblende (Bisilicafe of various proto rides and ^jer- 



oxides.) 



The common black amphibole, containing a'uminium, 

 or paragasite, with the non-aluminous species tre- 

 molite and actinolite, have already been recorded 

 (Catalogue of the "Minerals of Tasmania, 1896"), 

 but there are several others occurring here which 

 have hot so far been satisfactorily identified. 



At the Hampshire Hills a remarkably developed black 

 amphibole occurs. It is in large crystals, which often 

 reach several inches in length, and is closely asso- 

 ciated with pyrophyllite and amethyst. In thin sec- 

 tion under the microscope it is dark sombre green 

 and yellowish-green according to the orientation. In- 

 tensely pleochroic r = i) > ^ a. Extinction angle about 

 140. Crystallisation irregularly prismatic and flaky, 



