580 THE DISTRIBUTION, ETC., OP ALKALINE ROCKS, 



Mr. A. Gibb Maitland has also described trachyte tuffs in the 

 Upper Cretaceous west of Mackay.* 



All along the Queensland coast the trachytes have in general 

 an older appearance than inland in New South Wales. The 

 reasons are obvious. All recent sediments, all soft tuffs, and 

 many of the more decomposable lavas have been totally removed 

 by the rapid erosion due to a moist tropical climate. In some 

 places, as at Mt. Cooran, the trachytes intrude very ancient 

 granites, and if it were not the case that other neighbouring 

 masses very definitely intrude the Trias-Jura, we might mistake 

 these masses to be extremely ancient. 



As I have mentioned elsewhere, the removal of about 400 feet 

 of Trias-Jura strata in the Glass House Mountains proper would 

 probably leave all the masses surrounded by very ancient rocks, 

 and no one would be able to say that they were later than 

 Palaeozoic. 



Ch. v. 



We have now seen that alkaline rocks everywhere in Australia 

 exhibit many characteristics in common. 



The succession is in general from the more acid to the more 

 basic. In some cases a more basic intrusion has in Post-Triassic 

 times preceded the acid series, as the Dingo Creek dolerites in 

 the Nandewars, N.S.W., and the Point Arkwright porphyrites 

 in the Maroochy district, Queensland. 



In every case the alkaline rocks occur along the borders of the 

 Mesozoic transgressions and faulted continental masses, and they 

 always belong to the Eocene, a period of great earth-movements. 



The existence of cross-cracks, not unlike radial and tangential 

 cracks, as at Fonseca Bay, has been demonstrated, so that at 

 this time the volcanic portions of Australia were probably being 

 subjected to changes similar to those going on to-day in the 

 environs of the Gulf of Mexico. The alkaline volcanic areas 

 seem in general to be situated on subsiding portions of a Vorland 



* " Geological Features and Mineral Resources of the Mackay District," by 

 A. Gibb Maitland. By Authority, Brisbane, 1889. 



