BY H. I. JENSEN. 851 



the viscous masses must have remained in the place where 

 erupted, taking the form of mamelons.* 



An inspection of the arrangement of the columns on Ngun 

 Ngun and Tibrogargan clearly proves that the Glass House 

 Mountains are not the remains of a huge lava sheet, as has 

 been suggested by some. In most instances we see no evidence 

 of the lava having flowed from the vent. At Mt. Ngun Ngun, 

 however, very short lava flows have taken place. 



The rock is typically trachyte. In some places it is so coarsely 

 porphyritic as to become a felspar porphyry, as for example, the 

 Mt. Beerburrum rock. 



The felspar is universally of two generations, in phenocrysts 

 often somewhat corroded, and in minute laths forming with 

 iegerine the raicrocrystalline to cryptocrystalline base. Hence 

 it is probable that the magma had already cooled and partly 

 crystallised out at considerable depth, before it found vents and 

 broke through to the surface. 



The trachytic rocks are later than the sandstones (Ipswich- 

 Burrum Coal Measures), as proved by the following facts : — 



(1) The sandstones are traversed in various places by trachyte 

 dykes. 



(2) At the junction of the trachyte and sandstone the latter 

 shows unmistakable signs of metamorphosis, such as hardening, 

 induced crystallisation, and assumption of columnar structure. 



(3) Small tongues of trachyte have been injected into the 

 sandstones on the junction line. 



(4) The sandstones have been disrupted and sometimes tilted 

 at considerable angles by the trachyte. One good instance of 

 this is afforded by an anticline caused by a trachyte dyke in a 

 railway cutting about half a mile north of Beerburrum Station 

 (Plate xlix., fig. 5). At Mt. Beerwah and Mt. Conowrin the 

 sandstones dip away from the trachyte mass. 



* This is often the case with trachytic lavas. Compare the Puys of 

 Auvergne, the phonolite hills of Bohemia, and the mamelons of the Isle of 

 Bourbon. See "Volcanoes," by Jucld, ch. v. 



