ABSTRACT OP PROCEEDINGS. XXV. 



volcanic activity and periods of comparative quiet. The 

 great volcanic periods correspond with the periods of great 

 disturbance, that is great uplifts over extensive areas and 

 great subsidences in others. Volcanoes in any given region 

 inaugurate a period of uplift, but are accompanied by local 

 subsidences. The volcanic periods in Australia were the 

 Archaean and Precambrian, the Upper Devonian and Lower 

 Carboniferous, the Permo Carboniferous and the Tertiary. 

 The periods of comparative quiet were the Cambrian, Ordo- 

 vician, Silurian, Lower Devonian, and Upper Carboniferous, 

 the whole of the Mesozoic and the Post Tertiary. The 

 Tertiary period of activity is that of most importance both 

 geographically and economically. Extensive flows of basalt 

 form vast tablelands and plains, and give the most fertile 

 soils of the continent. The earliest Tertiary eruptions 

 were highly alkaline and form an interesting study. The 

 abrupt peaks and gigantic monoliths of the Glass Mountains, 

 Queensland, rising precipitously from a coastal plain, form 

 a wonderful sight. Similar extrusions initiated volcanic 

 activity in the Warrumbungles and Nandewars of N.S.W., 

 but in these districts they were followed by outpourings of 

 more fluid lavas so that a dome shaped mass of lava about 

 80 miles across was formed in the case of the Warrum- 

 bungles and an oval mass in the case of the Nandewars. 

 These were subsequently dissected by erosion. The alka- 

 line rocks follow a line which throughout geological time 

 has been a hinge line in all great earth movements, the 

 continent being alternately east and west of this line. 



To the great Carboniferous volcanic period we owe most 

 of our metallic mineral resources. This is well exemplified 

 on the South Coast of N.S. Wales, where the metallic lodes 

 are connected with diorite intrusions, and they occur 

 intruded into all rocks from the Ordovician to the Upper 

 Devonian. No Carboniferous rocks occur, but the over- 



