3o6 A NATURALIST IN THE PACIFIC chap. 



Note on the Rhombic Pyroxene of the three foregoing sub-classes 

 of the Acid Andesites. — The term " hypersthene " has been here 

 used as a convenient expression equivalent to " rhombic pyroxene." 

 The mineral is always a little pleochroic and is never colourless, 

 and it is only in very rare cases that the term " enstatite " could be 

 used. As a matter of fact there is practically only one form of 

 rhombic pyroxene represented in my collections whether in acid 

 or basic andesites or in hemi-crystalline and plutonic rocks. In 

 the acid andesites it occurs not only as phenocrysts but also as 

 minute prisms forming a constituent of the groundmass. 



This mineral, when composing the phenocrysts, presents itself 

 usually as single untwinned prisms which exhibit the typical octa- 

 gonal cross-sections with much reduced prism-faces. The prismatic 

 sections give straight extinctions ; whilst with the cross-sections 

 we obtain straight extinctions parallel with the pinakoid faces. 

 The colour in transmitted light is pale brownish yellow. The 

 pleochroism, though usually feeble, is quite distinct, the colour 

 being pale yellow, when the prism lies parallel with the long 

 axis of the lower nicol, and almost white when it lies across. 

 Not infrequently these phenocrysts behave abnormally and give 

 small oblique extinctions. This is often the case when monoclinic 

 pyroxene occurs in the same section. The association of the 

 two pyroxenes in one crystal can in some cases be clearly 

 recognised. At one time a plate of pyroxene exhibits itself as a 

 coarse aggregate of the two pyroxenes. At other times the two 

 occur as parallel intergrowths, as in the accompanying figure. 

 But it is rarely that such intergrowths are so typically displayed, 

 the reason of which has been supplied by Zirkel in his Lehrbiich 

 der Petrographie ; 2nd edit. : I. 271. 



l^ote on the " magmatic paramorphism " ^ of the hornblende 

 phenocrysts. — Reference has before been made in the general 

 description of these rocks to the dark alteration margins 

 of the hornblende phenocrysts. The dark borders display the 

 "bacillary" structure noticed by Renard in the case of some 

 hornblende andesites from Kandavu in the same group of islands,^ 

 being composed of minute granules and parallel prisms of 

 pyroxene and also of magnetite grains. With the Kandavu 

 rocks Renard observed that the tiny pyroxenes were colourless or 



1 I have borrowed this term from Rosenbusch's Microscopical Physiography 

 of the Rock-making Minerals, translated by Iddin^s. 

 ' Challenger Reports y Physics and Chemistry II. 



