,S50 GEOLOiiV OF (iLASS HOUSP: MTS. AND DISTRICT, 



gically i-eceiit limes a vast estuary into which a river flowed, 

 depositini,' the a])Ove-inentioned clays and gravels. On account 

 of tlie great variation in the thickness of the clay and pebble 

 layers indicating their j^urely local development in places where 

 small streams have formerly existed, as well as for other reasons 

 already recorded, I cannot entertain " the river drift " theory. 

 My observations on the formation of ironstone pebbles further- 

 more lead to a difterent conclusion. 



Rounded ferruginous pebbles were found in great abundance 

 under that grey ashy soil formed from decomposed trachyte 

 or trachyte tuff. On breaking them a core of trachyte 

 was frequently found. In the vicinity of Mt. Conowrin, Mt. 

 Beerburrum, and Mt. Miketeebumulgrai trachyte in course 

 of weathering has been observed to disintegrate into rounded 

 lumps covered with a ferruginous crust. This is particularly 

 the case with trachytes rich in deep blue pleochroic amphi- 

 boles. It seems, therefore, that, in many cases, the pebbles can be 

 traced to the decomposition of trachyte or trachyte tuff; it is very 

 probable that in early Tertiary times a great portion of the district 

 was covered with loosel}' cemented volcanic ashes and bombs. In 

 localities where ferruginous pebbles are in course of formation, 

 the decomposing trachyte has a nodular or tuberculate appearance. 

 These pebbles are nearly all less than half an inch in diameter. 



Another source of ironstone pebbles and nodules (from h to 3 

 inches in diameter) is the decomposition of the ferruginous 

 sandstones of the Coal Measure Formation. In the sandstones, 

 heavy ironstone concretions are abundant, and are left behind 

 when the rock containing them has weathered away. 



iv. Geology. 



(1). Structure and Origin of the Glass House Mountains. — 

 These mountains represent old volcanic plugs of trachytic lava 

 which have forced their way into ancient tuff cones now denuded, 

 or else have burst through fissures in the Coal Measure sandstone, 

 reaching the surface in a very viscous state. In the latter case 



