THE DESEET SANDSTONE. 331 



to have played the most important part in determining the 

 elevation and present physical outline of north-eastern Australia. 

 The main outbursts of lava have taken place along the dividing 

 range, and these are generally due to the Pliocene disturbance. 

 " The southern areas namely, Peak and Darling Downs &c. are 

 older ao-reeino- with the Lower Volcanic of Victoria." 



o o 



The rock masses of both periods are basic in character and may 

 with rare exceptions be all grouped as dolerites. Mr. Allport says : 

 "This dolerite contains triclinic felspar, augite, magnetite, pseudo- 

 morphs after olivine. The felspar prisms are clear and transparent 

 and exhibit well the striae and bands of colour when examined in 

 polarized light. The augite occurs in small brown crystals and 

 grains ; it frequently contains black magnetite and is sometimes 

 slightly altered. The olivine has been completely altered to iron 

 oxide, and appears in the sections as bright red grains and crystals. 

 Pseudomorphs of quite similar character occur in the dolerites 

 and basalts of the coast of Antrim." 



Mr. Daintree speaks of an interstratified bed in the Upper 

 V olcanic of a highly siliceous rock in which one half the mass was 

 composed of quartz crystals arranged in a quartzose matrix. 



In two places the Desert Sandstone is seen resting on the older 

 Volcanic, namely at Agate Creek, a tributary of the Gilbert River^ 

 and near Morinish station. 



These two volcanic periods formed a time when a fire-belt fissure 

 undoubtedly existed round nearly the whole of the continent of 

 Australia. On the south side there has been elevation of the land 

 to the extent of 600 feet, about the period of volcanic activity. 

 There are no such signs of elevation in any other part of the 

 continent ; no Tertiary marine fossils are found except on the 

 south side. The volcanic evidences are not confined to the main- 

 land : a few island craters exist at some short distance from the 

 coast-line ; such as Mount Prudhon, and probably several other 

 islets between Keppel Bay and Whitsunday Passage. There are 

 also flat-topped islands of Desert Sandstone amongst the numerous 

 groups in the same locality. Altogether the evidence on East 

 Australia is in favour of subsidence. On North Australia the 

 shallow sea and other evidence would seem to indicate a stationary 

 period. I do not speculate further on the geological history of 

 those times, for the absence of facts would make such guess-work 

 practically useless. The elevation of the Great Australian Bight 

 is, however, a fact we can point to with certainty that it must 

 considerably have altered the outline of the continent. This with 

 the volcanic period and the formation of the Desert Sandstone 

 are grand events in the physical history of our continent, succeed- 

 ing the great Cretaceous sea which once invaded the north- 

 eastern part. The fire-belt fissure seems to have been an isolated 



