BY W. F. PETTERD, C.M.Z.S. 133 



eral as '' a peculiar weakly bi-refringent monoclinic amphi- 

 bole, bluish-green in colour ; a grey-green, t brownish- 

 green, C bluish-green to blue, 2 E (the apparent optical 

 axial angle) small, optical character + and with strong 

 dispersion of the axes. It recalls strongly the blue-green 

 amphiboles of the crystalline schists in the Scora Vale, in 

 the centre and north of Norway, and elsewhere." 



The series of phonolitic rocks of Port Cygnet afford one, 

 and perhaps two, species of soda amphibole, but their specific 

 identification is attended with considerable difficulty. 

 Many of the rocks referred to have been microscopically 

 examined by Professor Rosenbusch, and that well-known 

 authority, in a communication to Mr. W. H. Twelvetrees, 

 refers to one of the hornblendes as being barkevikitic. 

 This is the prevailing form which is seen in rock sections 

 from the locality indicated. It is myrtle-green in colour 

 by transmitted light, and in the absorption t) > C > a, in 

 this respect appertaining more to katoforite, but differing 

 in the pleochroism. In the fractures and joints of the 

 elseolite syenite from the same place a black amphibole is 

 occasionally developed, having much the general appear- 

 ance of arfvedsonite. It is usually plentiful, occurring as 

 long narrow thin laths and aggregates, sometimes reaching 

 a trifle over 2 inches in length ; they do not show termina- 

 tions, but have an irregular brittle structure. Fragments 

 examined under the microscope show the substance to be 

 green by transmitted light. It is apparent that the soda 

 hornblendes at Port Cygnet differ in some degree from 

 those recorded from similar rocks in better known local- 

 ities, and that they require further investigation before 

 they can be satisfactorily determined. 



Specimens of the alkaline rocks of Port Cygnet have 

 been submitted by Mr. W. H. Twelvetrees to Professor 

 W. C. Brogger, of Kristiana, with a request for that 

 authority's opinion as to the definition of the hornblende 

 therein contained. The Professor, in reply, states : '' The 

 hornblende is certainly a peculiar alkali hornblende, which 

 I have found quite similar in various Norwegian nord- 

 markites. The material is not sufficient for an exact 

 examination. The weak absorption with light bluish- 

 green as principal colour, as well as the large extinction 

 angle, showed the hornblende would probably contain a 

 fair proportion of a glaucophane molecule in association 

 with a riebeckite molecule of less quantity. Titanium 

 contents must be small, though certain sections remind 

 m^ a little of hornblendes which pass into the katophor- 

 ites." 



