BY H I. JENSEN. 143 



The black (in thin slices blue-green) hornblende is found in 

 three habits — (a) corroded and indistinctly terminated pheno- 

 crysts; (b) in small columnar or rod-shaped crystals; and (c) in 

 arborescent aggregates of rhomboidal grains, all of which are in 

 optical continuity in each aggregate (pcecilitic habit). The first 

 type(a)was found to be best developed in Ngun-Ngun pantellarite, 

 and the phenocrysts were found to be of sufficient size and to show 

 cleavages sufficiently well to permit of an optical investigation. 

 The second type appears from the pleochroism (deep blue to 

 greenish-blue) to be riebeckite. It is developed together with 

 ^egirine and some arfvedsonite in the comendites of Conowrin, 

 Cooran, Coolum and Cooroora. In lath-shaped sections it appears 

 almost black. The third type is commonly associated with 

 cossyrite of similar habit, and appears from the pleochroism to 

 be riebeckite. It is developed in comendites and pantellarites 

 from Mt. Tibrogargan, Skeleton Cave (Trachyte Range) and the 

 N.E. side of Mt. JBeerburrum. It also occurs in some varieties 

 of Coolum comendite. The minute investigation of the first 

 type resulted in the following properties being revealed. The 

 hornblende is greenish-blue in thin slices, opaque in thicker 

 sections The D.R. is 0027; R.I. 1-7. The cleavage is 

 prismatic, perfect, at 56° nearly. Extinction angle on b with c 

 14°. Habit, tabular parallel with b. Pleochroism strongly 

 marked. Sections parallel to c from deep lavender to greenish- 

 yellow, parallel to b olive-green, parallel to a deep blue or 

 greenish-blue. 



C = c deep blue. 



a = a (nearly) light green, greenish-yellow. 



b = 6 lavender. 



Absorption c> b > a. 



These properties determine it to be arfvedsonite. 



In Ngun-Ngun pantellarite this amphibole often forms a border 

 round deep brown cossyrite. Occasionally it includes magnetite, 

 sometimes guarinite (1). In Beerwah trachyte, and more rarely in 

 Conowrin comendite, the blue hornblende has a nucleus of 

 brownish barkevicite (pleochroism brown to brownish-green). 



