BY E. 0. MARKS, B.A., B.E. 145 



<ioubt that some are Trias- Jura, excluding the Brisbane 

 tuff, the age of which has long been beyond question. 



There is little if any room for doubt also as to the 

 Trias- Jura age of much of the basaltic rock, more especially 

 in the eastern portion of the Macpherson Range. 



Opinion is divided in regard to the Toowoomba basalt, 

 no definite proof having been recorded up to the present 

 in favour of either view. 



Intimately associated as the volcanic rocks are with 

 the Trias-Jura system, it is essential to realise how much 

 or how little we know about that system before we refer 

 rocks to any particular stage of it. Up to the present time, 

 the only connected or in any way detailed examination 

 made covers a comparatively small area in the neighbour- 

 hood of Ipswich and Brisbane, and from there south to 

 Beaudesert — a total area of roughly some 1,000 square 

 miles of the strata. The whole formation covers at least 

 15,000 square miles, so our knowledge must be regarded 

 as very incomplete. Inside our limited area we are aware 

 of three divisions — in ascending order : — The Ipswich 

 coal measures, the Bundamba sandstones, and the Walloon 

 coal measures. We are, however, in no way justified in 

 supposing that the Walloon coal measures form the summit 

 of the formation, just as the Ipswich coal measures are 

 not necessarily the lowest division, though certainly so 

 locally. 



In the greater part of the areas in which the volcanic 

 rocks uccur, the sedimentaries have been the subject of 

 only few and disconnected observations, and it is highly 

 desirable to realise that, in the absence of connected and 

 reasonably detailed work, we are practically in con^plete 

 ignorance as to the stratigraphical position of these 

 sedimentaries in the Trias- Jura system. For instance, 

 it is a pure assumption, but one that is often made, that 

 the coal measures of the Darling Downs are on the same 

 horizon as those of Walloon. Except for a certain 

 similarity in their coals, there is really not the slightest 

 reason in our present knowledge to suppose this to be the 

 case, though there is every likelihood of it being so. 



The basalt of Tambourine Mountain is similar to, and, 

 except for denudation, continuous with that of the 

 Macpherson Range, which is almost certainly of Trias- 



