BY H. I. JENSEN. 589 



more fine-grained, so that it has not been found possible to deter- 

 mine them with accuracy. However, they agree with the 

 orthophyric type in having the a in the direction of greatest 

 elongation, and in the extinction angle (c : a) in that direction 

 being very slight (0°-5°). Carlsbad twinning is the commonest 

 variety of twinning. Probably this felspar is also anorthoclase. 

 It is the most abundant mineral both in the segirine trachytes 

 and soda-amphibole trachytes, and occurs as well in the trachy- 

 dolerites. 



Tabular Felsjoar. — This variety constitutes most of the pheno- 

 crysts in all the trachytes. It is tabular parallel to c. In other 

 respects the properties are similar to those already described for 

 the felspar of prismatic habit. 



Inclusions of a black titanium mineral, pseudobrookite (*?) or 

 Tutile (?), occur sparingly in the felspar phenocrysts. 



Soda-amphibole. — This is scattered about throughout the rocks, 

 l)earing it in irregular fragments, few of which are large enough 

 to give any indication of cleavage. In a few rock-types the 

 grains are united into dendritic (poikilitic) aggregates. Some- 

 times the hornblende occurs in the form of very dark, blue-blacky 

 rod-shaped crystals and irregular grains. The absorption is 

 greatest in the direction of the length of the rods, just as is the 

 case in the prismatic riebeckites of the rocks of Mt. Conowrin in 

 the Glass House Mountains, and Mt. Jellore near Mittagong.* 



In the more decomposed specimens the grains and rods are 

 replaced b}'- a reddish-brown nonpleochroic mineral {ferrite). 



In sections showing the two cleavages (at 56°), that is, perpen- 

 dicular to the c axis, the colour changes from deep indigo-blue to 

 blue-black, opaque, on rotating the stage. In sections parallel 

 to the face a the pleochroism is from deep blue to greenish-blue, 

 and parallel to the face b from greenish-blue to yellowish-green. 

 The extinction angle is about 14° (probably c : a). 



* Taylor &Mawson, ' Geology of Mittagong,' Journ. Proc. Roy. Soe. N. S. 

 Wales, Vol. xxxvii. 



