BY H. I. JENSEN. 155 



the base; whilst the riebeckite occurs in grains which consolidated 

 at the same time as the isometric phenocrysts. In others the 

 riebeckite and brown amphibole separated out together, the 

 latter, perhaps, commencing first. In such rocks the brown 

 amphibole may form the nucleus of arf vedsonite phenocrysts, and 

 also occur in dendritic aggregates in the groundmass (cp. Beerwah 

 and Ngun-Ngun rocks, pp.140, 145). 



In this connection it should be mentioned that most of the 

 orthophyric rocks are, however compact in appearance, minutely 

 vesicular (microlitic). Heating by means of a fire makes them 

 split, crack, an<l fly asunder with explosive violence, as I have 

 frequently experienced while camping out. The minute vesicles 

 probably contain water. The analyses show the presence of 

 zirconium, chlorine, fluorine, titanium, and sulphur. All this 

 points to extensive pneumatolytic action at the time of con- 

 solidation. 



Probably, owing to the viscosity of the lava, the water and 

 mineralisers were retained by it; and alterations in the relative 

 proportions of the mineralisers during crystallisation determined 

 the formation of different amphiboles at different times. The 

 mineralisers have also served to preserve eutectic proportions in 

 the magma during cooling. 



The Comendites of Coolum Mountain, near 

 P t. Ark vv right . — Several types were met with, all closely 

 allied. 



Sp. A. (Plate xv., fig.26). This is a fine-grained, bluish-grey, 

 columnar rock. Under the microscope it is seen to be holo- 

 crystalline, fine-grained, but uneven, with a pilotaxitic fabric. 

 It contains sanidine in laths and grains, and a dark blue-black 

 amphibole, which is too opaque to show pleochroism, probably a 

 variety of riebeckite. The dark amphibole occurs in long rod- 

 shaped crystals. 



Sp. B. The handspecimen is like A. It is a holocrystalline, 

 somewhat poiphyritic rock, with a very even-grained micro- 

 crystalline base which has a granular (microgranitic) texture. 

 The phenocrysts are isometric and lath-shaped sanidines. The 



