XXIV, ABSTRACT OF PROCEEDINGS. 



commoner phenomena and its effects were dealt with in a 

 general way and illustrated with slides. The eruptions in 

 Samoa in 1902 and 1905-9 were then described. The 

 lecturer explained how viscid lavas give rise to steep cones 

 and often to precipitous monoliths, while fluid lavas give 

 rise to volcanoes with but gently sloping sides. As ex- 

 amples of viscid lavas he mentioned alkaline trachytes and 

 phonolites, and as typical of fluid lavas basalts and certain 

 rhyolites. Scoria cones generally have a higher angle of 

 slope than tuff cones. Fluid lavas are also more apt to 

 give rise to fissure eruptions. The basalts of the Darling 

 Downs of Queensland and Monaro of N.S.W. are probably 

 largely the result of fissure eruptions. Volcanoes are best 

 studied when only in feeble activity, or when quite extinct. 

 It is as hopeless to gain much information from a violently 

 active volcano about its structure as from a machine when 

 all its parts are going at top speed. In Australia we have 

 many extinct volcanoes and dissected remnants of volcanoes. 

 To them we owe our finest scenery and our greatest 

 economic resources. Those which still maintain the perfect 

 cone and crater form are very recent, since volcanic cones 

 are largely made up of easily eroded tuffs and scoriae. The 

 Tertiary volcanoes are greatly dissected and Pre-Tertiary 

 volcanoes are represented merely by stumps, necks or their 

 plutonic offshoots, such as dykes and laccolites. It is only 

 when Pre-Tertiary lavas have been buried in sedimentary 

 rocks that they remain to tell the tale. 



The existence of great volcanic activity in Palaeozoic 

 and Mesozoic times is therefore made known chiefly by the 

 granitic and gabbroic deepseated intrusions which were 

 injected into the rocks of the earth's crust simultaneously 

 with the outpouring of lava on the surface. By the study of 

 these masses and the lava flows in the sedimentary series we 

 know that there have in Australia been periods of intense 



