858 GEOLOGY OF GLASS HOUSE MTS. AND DISTRICT, 



few instances of vesicular structure, such as we should expect 

 from the presence of much water, and no glassy rock, which would 

 result from rapid cooling. 



Tt is, however, likeh' that the sea was not far off at the time 

 of the Glass House Mountain eruptions. 



From the coarse-grained nature of the Triassic sandstones of 

 the East Moreton district in the Glass House Mountain region 

 and the abundance of fossil wood contained, it appears that these 

 rocks were deposited in a wide estuary. Sedimentation may 

 have lasted well into Cretaceous times, but so far no Cretaceous 

 rocks ha\e been identified in this region, though further north 

 we have the Maryborough Beds overlying the Trias. When 

 sedimentation ceased, the strata were elevated through rise of 

 isogeotherms; at a somewhat later period — probably the end of 

 the Cretaceous — recooling and denudation had progressed far 

 enough to allow cracking of the sedimentary strata. Through 

 cracks thus formed the Glass House trachytes found an exit. 

 Subsequent folding of the topmost beds probably gave rise to the 

 D'Aguilar Range and the Blackall Ranges, and this folding was 

 probabl}^ accompanied by the andesitic and basaltic outpourings 

 of lava. 



In age the trachytes are probably Pre-Miocene. No definite 

 proof of age has been obtained, but the amount of denudation 

 which they have suffered and the almost total removal of tuff"- 

 beds and crater rings, if these ever existed, hint at considerable 

 antiquity. The same lack of good evidence of geological age 

 seems to hold for most Australian trachytes, but the consensus of 

 opinion amongst our geologists, based on the small amount of 

 evidence available, assigns to them a Cretaceo-Eocene age. This 

 also seems to hold best for the Glass House trachj^tes. 



The basaltic rocks of Mt. Mellum bear considerable petrological 

 resemblance to those of Tambourine Mountain, described by Mr. 

 Rands, late Government Geologist of Queensland.* 



"" Jack ct Etheridge, * Geology and Palffiontology of Queensland.' 



