VOLCANIC ACTION IN EASTERN AUSTRALIA. 401 



Rrisbane. The rock consists of a felspathic matrix with blebs of 

 quartz and some crystals of orthoclase felspar ; its color is generally 

 white or a yellowish-white, though in places it has green, purple, 

 or brown tints. It contains throvighout it small particles of 

 micaceous schist and quartz, which towards its base attain the size 

 <.f jDcbbles or boulders. It has also near its base pieces of silicified 

 and also of carbonised wood. 



Taking into consideration the nature of the rock, its being full of 

 angular particles of other rocks, the absence of any change in the 

 surrounding schists, and, lastly, its position relatively to the other 

 formations, I have come to the conclusion that it is a volcanic ash, 

 and not a porph3"ry, and that it lies at or near the base of the 

 Ipswich beds." It appeared to me that these tuffs in places must 

 be probably not less than 500ft. thick at Brisbane. Traced in a 

 .southerly direction they thin out rapidly. At Corinda, however, a 

 few miles further south-west, the tuffs are replaced by a sheet of 

 amygdaloidal lava of basic or intermediate composition. 



Mr. Jack has drawn my attention to the fact that on a much 

 higher horizon in the Ipswich coal measures, and above that of the 

 productive coal seams of Ipswich, a considerable thickness of con- 

 temporaneous lavas and volcanic ash are interstratiiied with the 

 coal measures. Probably at least three groups are represented. 

 Tiie horizon of the principal group is just above that of a strongly- 

 developed sandstone, which Mr. Jack considers may possibly be 

 the equivalent of the Hawkesbury Sandstone of New South Wales. 

 This series is typically developed at Toowoomba. Overlying this 

 volcanic series are the thin coal seams, interbedded with shaly 

 rocks, of Brackets, Gowrie, Jimbour and Clifton. 



These may be on the same horizon as the Wianamatta shales of 

 the Sydnej^ coal basin, and may therefore be of Upper Triassic or 

 of Jurassic age, as in Queensland they pass conformably under the 

 Boiling Downs formation (Lower Cretaceous). For the horizon of 

 this volcanic series see " Geology of Queensland," by B. L. Jack 

 and B. Etheridge, jun., Plate XLIX., Fig. 2, to which I am largely 

 indebted for the information on Plate XV., section 3. 



Mr. Jack considers the Toowoomba lavas somewhat uncon- 

 formable to the lj)swich Series both above and below it. 



Near Wycarbah, twenty-four miles from Bockhampton, on the 

 Central Bailway, tuffs and basaltic rocks are di^veloped homotaxial 

 with some portion of the Ipswich coal measures. In my previous 

 paper I stated that, with the exception of some comparatively 

 insignificant tuffaceous beds in the Desert Sandstone of Queensland, 

 which is considered to be of Upper Cretaceous age, there is no 

 evidence, as far as the author is aware, of contemporaneous volcanic 

 action in Australia in Mesozoic rocks of Jurassic or of Cretaceous 

 age. It is possible, as above suggested, that the Toowoomba 

 basalts may be Jurassic, and the fact should be mentioned that, 

 although the passage just quoted is true generally of the Queens- 

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