140 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



group themselves into stellate masses. {See Plate XIII., 

 fig. 2.) The minerals present are quartz, anorthoclase 

 and sanidine. The zeolite, stilbite, occurs in cracks 

 through the rock. The specific gravity is 2-61. Name: 

 Ehyolite. 



Alkaline Rocks. 



These rocks have been fully described by Jensen,^'' 

 and it is worthy of note that there is a very great differ- 

 ence both macroscopically and microscopically between these 

 rocks and the rhyolites already described in this paper. 



We may group these rocks under two headings: . 

 (a) Pantellerites ; 

 ^ (6) Comendites. 



Pa)ttellerites. — One finds these in the Glass House 

 Mountains district, at Mount Ngun-Ngun and the Trachyte 

 Range, and also in the neighbourhood of Mount Flinders. 

 They are usually greyish to bluish-green in colour and 

 often show in the hand-specimen dendritic and mossy 

 aggregates of tpgirine. The fabric may be trachytic, hyalo- 

 pilitic, pilotaxitic or orthophyric, and the minerals are 

 quartz, sanidine, anorthoclase, riebeckite, arfvedsonite as 

 well as rarer minerals. 



Comendites. — These are usually lighter in colour than 

 the above, and occur at Mounts Conowrin, Tibrogargan, 

 Beerburrum and Ewin in the Glass House Mountains area, 

 and also at Mount French. Jensen^^ records them from 

 Spicer's Peak and ]\Iount Mitchell, but the author has failed 

 to find them there. The fabric is microgranitic or ortho- 

 phyric, and the minerals, quartz, sanidine, soda-sanidine, 

 ^girine, riebeckite and arfvedsonite. 



These special alkaline types are very limited in their 

 occurrence and their total volume is insignificant when 

 compared with that of the more normal rhyolites in the 

 southern portion. 



Chemical Composition of Khyolific Rocks. 



(a) Snb-alkaline and intermediate types. — An exami- 

 nation of the analyses shows that there is a close 



="'Proc. Linn. Soc, N.S.W., 1906-1909. 

 '^ A.A.A.S., xii., Brisbane, 1909, p 253. 



