82 GEOLOGY OF MT. FLINDERS AND FASSIFERN DISTRICTS, Q , 



felspar. It shows an hexagonal cleavage like nepheline. It is in 

 all probability katapleite. In still smaller amount we have a 

 yellow amorphous mineral which stands in medium relief, and 

 has a very low double refraction. This does not show the peg- 

 structure of melilite, yet it may be melilite. It may also be 

 perhaps eudialite. Cancrinite occurs associated with nepheline 

 and katapleite, and is distinguished from the latter by its low 

 index of refraction. Complex twinning is another characteristic 

 of the katapleite. Nosean appears also to be present, associated 

 with the same decomposition-products. 



This rock contains veins of alum. The alum is derived from 

 the decomposition of some mineral like nosean, or from some 

 sulphosilicate like microsommite. 



From the nature of the contained minerals we must call this 

 rock "phonolitic breccia." 



2. The Trachytes, (a) Comendites. — Rocks belonging to this 

 family occur abundantly on Mt. Flinders, Mt. Blaine, and sur- 

 rounding pinnacles, and on Mt. French and on the Little Liver- 

 pool Range. The following has been selected for detailed 

 description. 



F1.17. Macroscopically, a fine-grained, aphanitic, white rock, 

 occurring as gigantic vertical columns extending to the summit 

 of Mt. French. 



Microscopic examination : texture, holocrystalline, microcrys- 

 talline with orthophyric fabric. Composition : the essential 

 minerals are felspar and quartz. The minor constituents comprise 

 arfvedsonite, secondary magnetite, peropkite(^), chalcedony, and 

 tridymiteC?). The felspar is chiefly of the isometric or almost 

 isometric form, though some is prismatic. It is the most abundant 

 mineral, amounting to about 70 % of the rock. It possesses the 

 usual properties of sanidine, and is partly idiomorphic and partly 

 allotriomorphic. Quartz comes next in order of abundance. It 

 occurs chiefly in the form of idiomorphic prisms, hexagonal in 

 section, and forms about 20 % of the rock. There is also present 

 allotriomorphic quartz and chalcedony. The chalcedony occurs 

 chiefly as irregular masses infilling cavities which were originally 



