40 Diorites and Granites of Sivifls Creek, 



the lustre of which is somewhat vitreous. It becomes 

 transparent in the thin slice, and is then either colourless or 

 faintly reddish. It is remarkably fresh and unaltered, and 

 is free from inclusions. Most usually the sections show a 

 longitudinal somewhat perfect cleavage ; occasionally a 

 slightly undulating, finely fibrous structure ; rarely (and this 

 seems to be in sections across the former) a prismatic 

 cleavage of nearly 90° and two pinacoidal cleavages, of which 

 one is more perfect and the other less perfect than the 

 former. The plane of vibration in the longitudinal sections 

 is parallel to the cleavage, and in the cross sections diagonal 

 to the prismatic cleavage. Absorption is not marked, but 

 the mineral is still polychoic. In the longitudinal sections 

 the dichroism is in light shades of red to colourless ; in the 

 sections across the prism in light shades of red and pale orange 

 red. These observations indicate an orthorhombic mineral, 

 having a perfect cleavage according to one pinacoid, a less 

 perfect prismatic cleavage of nearly 90°, and a least perfect 

 cleavage according to the other pinacoid. Assuming the 



most perfect cleavage as parallel to ii, the axis colours would 

 then be as follows : — a, pale red; b, pale orange red; c, grey or 

 colourless. The mineral is not affected by boiling with 

 hydrochloric acid. These observations agree exactly with 

 the characters of hypersthene as given by Rosenbusch and 

 Des Cloizeaux; but I have not observed the microscopic 

 plates which are generally regarded as characteristic of it. 

 The optical characters of this mineral place it in the ensta- 

 tite-hypersthene group ; and the question arises — " Have we 

 a dichroic enstatite, or a hypersthene without the micro- 

 structure of that mineral as found at Isle St. Paul ? I 

 should decidedly reply in the affirmative to the second part 

 of the question, for the pleochroism cannot be due to inclu- 

 sions, but may result from the composition of the mineral."* 

 I may note here that the only other mineral which might be 

 indicated by the orthorhombic character of this species 



would be andalusite. But the red ray in this case is c, so that 

 thin slices perpendicular to the prism remain colourless when 

 tested by the polariser alone. A second constant inclusion in 

 the amphibole-anthophy llite is olivine. It is found invariably 

 as rounded or elliptical grains. In the thin slice seen by ordi- 

 nary light they are colourless, free from inclusions excepting 



*Rosenbusch, Op. Oit., p. 479. 



