president's address — SECTION c. 67 



and the main axis of the Australian Cordillera determined. 

 Upheaval, therefore, followed after prolonged subsidence. 



At the close of the Lower Devonian age a great series of 

 terrestrial volcanoes broke out on a north-and-south line from 

 near the Cobboras southerly towards Tasmania. At the close 

 of the Lower Devonian age the land surface sank, but 

 volcanic activity did not entirely die out until deposition of 

 the Buchan limestones of Middle Devonian age. 



A considerable subsidence then took place, accompanied by 

 a complete cessation of volcanic activity. 



Finally, before the deposition of the Upper Devonian 

 series the Lower and Middle Devonian rocks were uplifted 

 and folded to such an extent that in some places in Victoria 

 the Middle Devonian rocks are vertical, whereas the over- 

 lying Upper Devonian rocks are horizontal and comparatively 

 uriViisturbed. 



The Upper Devonian rocks of the Avon River were next 

 deposited, and contemporaneously with their formation lavas 

 of a basic character were outpoured. These eruptions 

 probably took place on an area of subsidence, though the 

 downward movement must have ceased about the time when 

 the last of these melaphyre sheets was erupted, as freshwater 

 beds containing lepidodendrons conformably overlie the mela- 

 phyres. 



Very little folding appears to have taken place in the 

 Victorian rocks subsequent to the close of the Middle 

 Devonian period, the Upper Devonian and Carboniferous 

 rocks being only slightly upHfted and compressed. 



In New South Wales abundant evidence of contempo- 

 raneous volcanic action during the Carboniferous period is 

 afforded by the thick beds of diabasic and felsitic tuffs, 

 together with sheets of felsite and diabasic basalt interstrati- 

 fied with the Rhacopteris beds, which form the Upper division 

 of the Carboniferous system in New South Wales, the Lower 

 division being specially characterised by lepidodendrons. The 

 total thickness of the Upper portion of the Carboniferous 

 beds in the Stroud District of New South Wales is at least 

 ten thousand feet, and out of this probably not less than one- 

 third is composed of contemporaneous volcanic rocks, these 

 being the oldest known lavas and tuffs in JNew South Wales. 

 Little, however, is at present known of the conditions under 

 which these lavas and tuffs were formed, beyond the fact that 

 prior to their eruption a considerable thickness of marine 

 carboniferous sediments were deposited, and that the scene 



