BY H. I. JENSEN. 147 



Composition. — From the discussion of the minerals 

 present it is easily seen that the trachytes are highly alkaline 

 and rich in soda. The magma must also have contained zirconium, 

 titanium, and fluorine, as shown by the presence in the rocks of 

 zircon, sphene, guarinite (I), wohlerite (1) and topaz (?)■; chemical 

 analysis bears out this conclusion. 



As other rather rare constituents present in the trachytes, the 

 following deserve special mention: — zircon, sphene, apatite, 

 magnetite, hematite, and a yellow mineral in small grains. The 

 last-mentioned mineral shows rather high interference colours, 

 reaching yellows and reds of the first order, and is probably 

 marialite (a scapolite mineral). Some of the chlorine (see chem. 

 analyses) is probably contained in it. Minerals resembling 

 wohlerite, mosandrite, and rinkite have also been noticed, but 

 have not been identified with certainty. 



Brown mica occurs in the trachy-rhyolites from the Green 

 Hills (Pt. Arkwright) and Mt. Peregian. 



The Trachyte-like Alkaline Volcanic Rocks. — 

 In handspecimen these rocks are very much alike. They are 

 light grey in colour, and finely speckled with deep blue-black 

 amphibole. Often they are full of minute vesicles, a characteristic 

 which is only noticed by the extreme lightness of some specimens 



(a) Those from the Glass House Mountains: — 



(i.) Beerwah Trachyte (Plate xiv. fig. 19). One variety was 

 described in my previous paper. The variety now described also 

 occurs in shingly slabs, and has a bluish colour due to an 

 abundance of blue hornblende. It has a silky lustre. 



Microscopic examination. — (1) Texture : (a) crystallinity, non- 

 crystalline; (b) grain-size, fine aphanitic, but with a few phenocrysts 

 of sanidine and arfvedsonite; (c) fabric, pilotaxitic with a strong 

 tendency to flow structure; hence the fabric is really trachytic. 



(2) Constituents (in order of decreasing abundance) — (a) 

 felspar, (b) greenish-blue hornblende, (c) segirine, (d) quartz, (e) 

 garnet (?),' (f) magnetite, (g) zircon, (h) apatite. 



(3) The felspar is sanidine, and perhaps partly anorthoclase or 

 wholly so. It occurs in small laths forming the main bulk of the 



