144 THE GEOLOGICAL AGE OF VOLCANIC ACTIVITY 



(2) A number of trachyte pebbles found in conglom- 

 erate in the upper portion of the Walloon coal 

 measures, in the Fassifern and Lockyer districts. 



The authors thus ascribe the trachytic as well as 

 basaltic rocks to two distinct ages. The Toowoomba 

 basalt, an olivine basalt, is considered to be Tertiary, 

 being distinct petrologically from the basalt referred to as 

 Trias-Jura age, which appears to be of a similar character 

 to that of the Upper Logan districts. 



It is interesting to note that the numerous areas of 

 basaltic lava which occur in the neighbourhood of Brisbane, 

 still occupying in places the valleys in which it originally 

 flowed, and nowhere attaining any noteworthy elevation, 

 consist of an ophitic dolerite varying very slightly in 

 character in any of the localities from which the MTiter 

 has examined specimens. This rock, which is of 

 undoubted Tertiary or post-Tertiary age, is equally distinct 

 petrologically from either the Toowoomba basalt or the 

 andesitic basalt of the Macpherson Range. 



This dolerite is familiar to us as the " blue metal " 

 of the streets, for which purpose its great toughness is 

 somewhat discounted by its defective binding power — 

 a defect due to its fine and uniform texture. At Redbank 

 Plains, Cooper's Plains and Runcorn, and probably else- 

 where, this rock lies on strata, bearing, in certain localities, 

 a plentiful dicotyledonous flora as aa ell as fish and reptilian 

 remains, the age of which have not yet been definitely 

 determined. 



From the foregoing list of observations and opinions, 

 we may summarise the present state of our knowledge 

 of the ages of vulcanicity as follows : — 



Undoubted post Trias-Jura basaltic rocks occur near 

 Brisbane, and at Clifton, on the Darling Downs. These 

 rocks are probably of late Tertiary if not of post-Tertiary 

 age. 



From opinions formed in the field, though not sup- 

 ported up to the present by any positive proof, several 

 observers are persuaded also of the post-Trias-Jura age 

 of some (or most) of the trachytic rocks, a diversity of 

 opinion existing as to whether the main body of these are 

 Trias-Jura or post-Trias-Jura. There can be no reasonable 



