8-54 GEOLOGY OF GLASS HOUSE MTS. AND DISTRICT, 



cemented tult^ the coarsely crystalline nature of the rock and the 

 viscosity of trachytic magmas generally, supporting such a sup- 

 position. It could also be interpreted as a large dyke mass. It 

 is probably not a flow from an^' of the adjacent trachj'te 

 mountains, the rock being macrocrystalline and porphyritic. 



(5). Occurrence of Tufaceous Rocks. — The occurrence of un- 

 doubted tuffs is very rare. Tuffs mai/ formerly have covered a 

 large area and formed cones round the trachyte plugs, but must 

 have been very loosely cemented. Hence it seems improbable 

 that the Glass House Mountains were submarine volcanoes, sub- 

 marine tuffs being usually fairly compact. The only undoubted 

 tuffs observed were those of a ridge to the south of Mt. Tibro- 

 gargan which I have called the Trachyte Range.* At a spot on 

 this ridge — Skeleton Cave, south of Mt, Ewin — where I dis- 

 covered some aboriginal skeletons in a cave, p3^roclastic rocks 

 with large angular fragments occur. Some specimens obtained 

 on Tibrogargan may be tufaceous, but have not yet been properly 

 examined. 



Trachyte Range is a low continuous ridge of trachyte (rising 

 in some places to an altitude of 300-350 feet above the surround- 

 ing country). It runs from Mt. Beerburrum in the direction of 

 Mt. Tibrogargan, taking, however, a westerly trend at a place to 

 the S.S.E. of Mt. Ewin, and continuing almost to the Gympie 

 road. The core of the ridge consists of compact trachytic lava 

 similar to that of Mt. Ewin, and also very like that of Mt. Jellore, 

 of which Mr. T. G. Taylor, of the Sydney University, has kindly 

 shown me some sections. At various points, as at Skeleton 

 Cave, tuffs occur, these forming a hard, greenish rock, emitting 

 a ringing sound when struck. They have evidently been highly 

 silicified. The entire ridge evidently marks an earth-fissure 

 which has emitted lavas and tuffs. Probably siliceous hot springs 



* The names, Trachyte Range, Skeleton Cave, Mt. Bokay, and Mt. Cooee, 

 made use of in this paper, refer to localities which frequently require mention, 

 but which have not as yet local names, nor do they bear names on the official 

 maps. 



