80 GEOLOGY OF MT. FLINDERS AND FASSIFERN DISTRICTS, Q., 



tell the tale of volcanic eruptions. Only the plugs of extinct 

 volcanoes would have been left. 



Being a low, inland plain, situated between the Palaeozoic 

 coastal hilly country and the upraised Darling Downs, the 

 Ipswich-Fassifern district was protected from extensive Tertiary 

 erosion. 



We see, then, that the country east of the Darling Downs has 

 probably been depressed by early Tertiary trough-faulting. The 

 Ipswich area was greatly fractured and overthrust. The area 

 covered by the Walloon coal-measures subsided in a block, and, 

 being practically a broad, belted coastal plain at the outset (in 

 the late Cretaceous), and being by progressive subsidence lowered 

 to the new base-level before its configuration could be interfered 

 with by eastward-flowing streams, it has preserved an old and 

 mature appearance which is only disturbed by Tertiary lava- 

 domes, and Tertiary folding in the Ipswich beds. 



Petrology. 



1. Pyroclastic Rocks. — In the Little Liverpool Range a trachyte- 

 agglomerate or breccia, with boulders up to many tons in weight, 

 is a rock of common occurrence. Basic tuff's and breccias have 

 also been observed in places. At Mount Flinders and on the 

 slopes of Mt. Blaine considerable areas are covered with a breccia 

 intermingled with lava which has a brecciated structure, having 

 swept up fragments of the underlying breccia. Some of the 

 breccias are distinctly trachytic, others appear andesitic. 



The lavas interstratified with the breccias on Mt. Flinders and 

 Mt. Blaine look like the dacites of Bankfoot House in the Glass 

 House Mountains. Quartz and oligoclase are common constituents 

 in them. Zeolites, pseudobrookite, magnetite, and ilmenite also 

 occur. In addition, the usual trachyte-felspars, sanidine and 

 anorthoclase, are quite common, and the groundmass is generally 

 a dark, often black, cryptocrystalline to glassy lava, frequently 

 with microspherulitic structure well developed in it. 



Globulites, margarites, axiolites, crystallites, and spherulites 

 are very common structures in these hypohyaline brecciated lavas. 



