118 E. 0. Teale: 



The largest felsj^ar phenocrysts are inclined to be of rather stout 

 habit, showing somewhat broad twin lamellae, and generally low. 

 extinctions, suggesting oligoclase. 



The numerous felspar laths, which Ijulk most largely in tlie slide,.. 

 and show simple twinning, are probably sanidine. as also are a. 

 number of definitely prismatic form.s of intermediate size. Mag- 

 netite is abundant, but almost all trace of the original ferro-mag- 

 nesian mineral has disappeared. Veins and patches of secondar;v'^ 

 quartz and chlorite are present, and one portion of the slide has 

 been almost completely silicified, showing the quartz both as av 

 mosaic and microcrystalline form stained by patches of hematite 

 and some chlorite. 



The rock appears to represent an interesting type of altered^ 

 trachyte or trachytic andesite. 



No. 14 is a dark grey, finely porphyritic rock macroscopically, 

 but the thin section shows a distinct fragmental structure, with- 

 abundant angular fragments of triclinic felspar (probably an- 

 albite of the type Abjg An J. Smaller felspar laths are present,, 

 some of which are triclinic, w^th low extinctions, and, maybe, oligo- 

 clase, also m,agnetite and fragments of altered rocks similar to 12; 

 much chloritized. The base is finely granular in part, due to second- 

 ary silicification. There are no recognisable original ferro-mag- 

 nesian minerals, but chlorite is abundant. The rock probably re- 

 jDresents an altered andesitic ash. 



Acid Porphyritic and Pi/roclasfic Boclis. — Rocks of this type- 

 are very abundant, and show «_onsiderable variety of colour aiid 

 texture. In general a prevailing red to brown colour is most com- 

 mon, but various shades of green to grey are also found. 



Flow structure is not common, and was only noted in one place, 

 namely, in the ridge between Ti-Tree Creek and Yellow Water Holes- 

 Creek, and W.N.W. fi-om Beecher's. 



Quartz and felspar either as fragments or phenocrysts are usually 

 appaz'ent macroscopically, and in thin sections the rocks of this- 

 division can generally bei readily separated into two groups — 



(1) Those definitely fragmental, representing altered pyro- 

 clastic material. 

 (2) Those €if the more normal quartz-porphyrite type, probably 

 partly hypabassal. having solidified in the fissures, or 

 vents, through which the more superficial materiaP 

 reached the surface. 



Roth these types are well represented along the Tara Range. 



