I 



liY H. I. JENSEN. 8G7 



The more fine-textured trachytes, such as those of the Round 

 Mountain, Mt. Conowrin, Mt. Evvin, and Mt. Tibrogargan contain 

 a greater proportion of segerine and less hornblende. Occasionally 

 crystals of deep blue, highly pleochroic riebeckite are present. 

 'J'he main constituent of all the trachytes, both coarse and fine, is 

 felspar, anorthoclase, with sanidine (orthoclase) in the Beerburrum 

 rock, andesine or oligoclase and sanidine in the Beerwah rock 

 (the plagioclase being here the more abundant constituent), and 

 sanidine, with or without some anorthoclase, in the other rocks; 

 the more basic minerals form but a minute portion of the bulk of 

 the rock. The hornblendes seem all to be soda-bearing varieties, 

 strongly pleochroic and deep blue, green or greenish-brown, 

 possessing strong absorption in certain directions, being allied to 

 the species riebeckite, arfvedsonite, barkevicite, and cossyrite (?). 

 The augite is chieli}" a soda-bearing variety, pegerine, in rods and 

 needles. 



Pilotaxitic and trachytic textures are seen in the rocks of Mt. 

 Beerwah, Round Mountain and in some of the Conowrin rock. 

 A microrthophyric base obtains in most of the other trachytes. 



Holocry stall inity is univeral in the trachytes, but the grain 

 size of the base varies from cryptocrystalline to microcrystalline. 

 Porphyritic structure is also prevalent. 



The felspar phenocr3'sts are usually somewhat corroded, though 

 sometimes perfectly idiomorphic; hence the felspar seems to be 

 of tiDO yenerations, partial crystallisation having taken place in a 

 subterranean reservoir, leading to the formation of the blue 

 hornblendes with strong absorption, the deep green hornblendes 

 (arfvedsonite), and many of the felspar phenocrj'sts. Partial 

 resorption has taken place in the upward passage of the magma. 



Zoning is common in the idiomorphic felspar phenocrysts. 



(u.) Andeslte ( Dacite) Formation at Grigor's Place. — This lava 

 varies immensely in composition, texture, colour, ikc. It covers 

 an area of about one scjuare mile, lying between Beerwah, 

 Conowrin, Tibrogargan and Ewin. The colour of the rock is for 

 the most part dark grey to black, but in the close ^■icinity of 

 Bankfoot House we find it — 



