JJY H. I. JKN'SEN. 593 



2. Microscopic texture : holocrystalline, with even-grHined 

 <;rystal8 cemented with a fine, even-grained, microcryBtalline base; 

 fabric trachytic. 



3. Composition : felspar forms upwards of 95 % of the mass; 

 it consists of sanidine-anorthoclase in lath-shaped crystals, in the 

 interstices between which the other minerals of the rock are 

 found. The most abundant soda amphibole present is of the 

 type already described. There is also present in larger ragged 

 grains a small amount of a brownish soda-am phi Vjole, highly 

 ploochroic in colours from a deep brownish-black to light h>rown, 

 purple, and even dark green. It is commonly surrounded by a 

 rim of blue amphiV>ole. It is proVmbly one of the highly titani- 

 ferous amphiboles of the cossyrite family. Frequently it occurs 

 in optically continuous aggregates enclosing felspar in a poikilitic 

 manner. It also seems to have affinities with katophorite. A 

 minute quantity of interstitial segirine is present; sometimes it 

 is gathered round arfvedsonite crystals. 



4. Order of consolidation : felspar commenced to crystallise 

 before the other minerals. The ferriferous minerals crystallised 

 simultaneously with the last of the felspar. 



5. Name : Trachytic Soda-Trachyte. Magmatic name, Nord- 

 makose (see Analysis W.38). 



Note. — The brown pleochroic amphibole mentioned above 

 occurs in many of the rocks of this area and the Nandewar 

 Mountains. Occasionally it is seen to shade off into clear 

 reddish-brown non-pleochroic ferrite, a substance which in many 

 rocks replaces it. At first I was under the impression that the 

 rocks containing ferrite and haematite were altered by weather- 

 ing, but a closer examination, aided by the chemical analyses, 

 has convinced me that the alteration is in most cases due to 

 mineralising vapours, and took place in the period of volcanic 

 extravasation and of the cooling of the magma. Not only do we 

 find all gradations from arfvedsonite to ferrite (including the 

 brown pleochroic amphiboles; in the same rocks, but the felspars 

 are quite fresh in many rocks in which ferrite is the predominant 

 coloured constituent, and decomposition-products are rare and 



