president's address SECTION c. 183 



precisely .defined, the possibility of the basal part of the series 

 beino- of U. Silurian age cannot be dismissed, especially in view 

 of the occurrence of volcanic rocks in that series near Yass, in New 

 South Wales. 



Petroloyical Characters. 



Veiy little microscopic examination of these rocks has been 

 undertaken. Howitt (4-3) has given a preliminary description of a few 

 rock sections, but a great deal remains to be done before any com- 

 prehensive account of the petrology of the series can be written. 

 Howitt has shoAvn that the oldest rocks consist mainly of lava flows, 

 while the later series consist chiefly of tuffs and agglomerates with 

 minor outpourings of lava. The lower rocks are described as quartz- 

 porphyries. Some of them with fluidal structure should probably be 

 grouped with the rhyolites. The upper more fragmental rocks he 

 refers to as felstone-porphyries, felstone ash, and agglomerates. 

 Many of the pwoclastic rocks have suffered secondary silicification, 

 a change which has more or less disguised the characteristic matrix 

 of tlie rock under the microscope. 



Some of the higher mountains in the Snowy River poi-phyry area 

 Howitt regards as the denuded centres of volcanic activity. In these 

 cases a central deeper-seated mass of quartz-porphyry is surrounded 

 by lava flows and pyroclastic rocks. 



The Cobberas, Wombargo Mountain, and Mount Hotham sho-w 

 this structure. 



Outside of the present area of the Snowy Kiver porphyries occur 

 a number of hills of granite^porphyry rising through older rocks. 

 Mount Taylor, Mount Alfred, and Mount Lookout, near Bairnsdale; 

 Mount Nowa Nowa, at the head of L. Tyers; and Mount Raymond, 

 east of the Snowy River mouth, are examples of hills of this type. 

 I'rom their general petrographical resemblance to the central por- 

 phyritic rocks of the Cobberas, &c., Howitt suggests that the above- 

 mentioned hills may represent the plugs of some of the volcanoe" 

 from which the Snowy River porphyries were derived. 



Clie?nical Characters. — These can only be inferred from the micro- 

 scopic determmations, as no analyses of this series appear to have 

 been made. 



LOWER DEVONIAN (?) 



The Alkali Rocks of Eastern Victoria. 



In three distinct areas in Eastern Victoria there occur rocks of an 

 alkaline character which have been described by Howitt, but which 

 have not hitherto been definitely placed among the alkali series. The 

 districts in which they occur are near Mount Leinster station, in 

 Benambra; Frenchman's Hill, near Omeo ; and Mount Elizabeth, 

 Noyang, near Avhere the road from Bruthen to Omeo first crosses the 

 R. Tambo. 



The Mount Leinster Rocks in Benambra. 

 Geographical and Geological Relations. 



Mount Leinster forms a prominent landmark in Benambra, rising 

 1,500 ft. above the surrounding country, and about 4,000 ft. above sea 

 level (58). It lies about 25 miles in a N.E. direction from Omeo, and 

 Mount Leinster station is situated to the north of its northern slopes. 



