140 THE GEOLOGICAL AGE OF VOLCANIC ACTIVITY 



hovi'ever, to discuss the petrology, except in so far as it 

 concerns the determination of the geological age. 



The rocks may be divided for field purposes into the 

 two main types, trachytic and basaltic, though each of 

 these contain widely differing rocks. Many of them have 

 been determined by Dr. Jensen to belong to the alkaline- 

 types. Both tuffs, lavas and dykes are met with. 



All Brisbane residents are familiar with the trachytic- 

 tuff, or so-called " porphyry," on the streets and in build- 

 ings, as well as with the rock in situ in Kangaroo Pointy 

 Leichhardt Street, and other places in and near the city. 

 Trachytic rocks also occur in or form the Glass House- 

 Mountains and other mountains in East Moreton, Flinders^ 

 Peak, Mts. Lindsay and Barney, Cunningham's Gap, the 

 Little Liverpool Range, and near Esk, besides many other 

 localities known, and many, probably, no£^ yet recorded. 



Rocks of the basaltic type are even more extensively 

 developed, oiccupying, as they do, so much larger an area^ 

 than the more acid lavas, due probably to their more 

 mobile condition when extruded. They partake in the 

 formation of the Blackall Range, Tambourine Mt., portions 

 of the Macpherson Range, Little Liverpool and Main Range, 

 as well as large part of the Darling Downs. They are 

 found besides at lesser elevations, and in the neighbourhood 

 of Brisbane and Ipswich, from such coir par atively low- 

 lying areas as Cooper's flains. Manly, Redbank Plains^ 

 Bundamba, etc. 



The first observer to offer an opinion as to the ages 

 of the volcanic rocks was the late Sir A. C. Gregory, who^ 

 in his capacity of Geological Surveyor for Southern 

 Queensland, between the years 1875 and 1879, traversed 

 much of the country in which they occur. In his report 

 on the south-eastern districts of the Colony of Queensland, 

 he says that the basaltic rock " may be referred to a very 

 recent date in the Tertiary era." He refers, both in this 

 report and in one on the coal deposits of West Moreton 

 and Darling Downs, to the fact that at Clifton coal mine, 

 under 30ft. of basalt, large pieces of fossil wood were found 

 so little altered that it split and warped several inches on 

 being exposed to the air. It was associated mth woody 

 seed vessels {Conchotheca Turgida). The latter were 

 referred for determination to Baron von Mueller, who 



