BY H. I. jensp:n. 871 



found {cf. The Glass House Mountains, aiitr); in structure they 

 are coarsely porphyritic, another point of resemblance to many of 

 the Glass House Mountains lavas {e (/., Beerburruni, Ngun-Ngun). 

 Fayalite-melilite basalt occurs not far away, at One-Tree Point, 

 and has been desLril)ed by Mr. Twelvetrees. Similarly in the 

 Glass House Mountains area we have the Fayalite Ijasalt of Mt. 

 Mellum. The age of the Port Cygnet trach3'tes is Upper Creta- 

 ceous or early Eocene, approximately the same as that of the 

 Mount Macedon rocks. 



The trachytic lavas and tiie syenites of the Mittagong district 

 have also during the last eighteen months received very thorough 

 investigation at the hands of Messrs. Mawson and Taylor, of the 

 Sydney University.'^ The Gib Rock syenite and neighbouring 

 trachytes have been shown by them to be probably UpjDer Cre- 

 taceous, at all events Post-Triassic and Pre-Tertiar}. In chemical 

 composition they have found it to be exceedingly rich in alkali 

 (Mawson). 



At Port Mackay, in Queensland, trachytic tufts are described 

 by Mr. A. Gibb Maitland as abundantly interstratified with 

 Desert Sandstone of Upper Cretaceous age.f 



At Yeppon, near Rockhampton, Q., a range of trachytic 

 mountains occurs. 



A large number of steeply conical mountains are interspersed 

 with more gently sloping (probably basaltic) mountains in low- 

 lying countr}^ south of the railway line between Brisbane and 

 Helidon (Main Southern Line). Many of these may yet prove 

 to be syenitic or trachytic in nature. 



Mr. Rands describes a mass of trachytes containing beautifully 

 developed sanidine crystals as occurring in railway' cuttings 

 between Logan village and Beaudesert, near Walton Station. 

 They seem to have come up through the Ipswich Coal Measures 



* Paper read before Royal Society, New South Wales, October 7, 1903. 



t "Geological Features and Mineral Resources of the Mackay District." 

 By Authority: Brisbane, 18S9. Also Jack & Etheridge, o}). cit.. Text 

 pp. 546-547, 1892. 



