president's address SECTION c. 209 



The basalts vary considerably in thickness from a few feet up to about 

 200 ft. Fresh and altered scoriaceous and dense types, fine and 

 coarse grained varieties are all repi'esented. Platy, spheroidal, and 

 columnar structures are well shown ; hexagonal pavements at Coburg, 

 near Melbourne, and the organ-pipe coknnns near Digger's Rest ar& 

 excellent examples of the last-mentioned structure. The geological 

 ages of the newer basalts vary considerably, probably from the Pliocene 

 to the recent, if not historic, periods. Professor Gregoiy has dis- 

 cussed (255) the question as to whether man witnessed the last enip- 

 tions of the Victorian volcanoes. The basalt covering, in places 

 liundreds of feet in thickness, which seals up the buried alluvial gravels 

 or deep leads, Avas poured out while the gravels were being formed. 

 Fossil leaves and fruits in these gravels have been referred to the Plio- 

 cene, and although these substances are not of high zonal value, and 

 the reference to a Pliocene age is not certain, yet the fi-uits, &c., are all 

 of extinct forms and indicate considerable antiquity for the gravels, 

 and presumably for the lavas, which sealed up the old valleys. Near 

 Ballarat four flows of basalt are met with in sinking shafts through 

 the leads, and intercalated between each are alluvial gravels. In 

 many areas the exact age of the basalts cannot be determined, since 

 they form the superficial rock. In these cases physiographic relations 

 afford some help. In some cases since the pouring out of the basalt- 

 sheets, and the consequent obliteration of former stream-courses, 

 the new rejuvenated streams have trenched the basalt plains with 

 river valleys of considerable depth and breadth. In some cases the 

 valleys are nearly 200 ft. deep, and many are V-shaped in cross-sec- 

 tion, indicating a still youthful stage of dissection. In other cases they 

 are as much as a mile in breadth and fairly mature. These latter must 

 be of considerable antiquity. The youngest volcanic rocks probably 

 occur at and near Tower Hill, west of Warrnambool. Road cuttingfs 

 on the Warrnambool road show, in places, that the tuffs overlie the 

 geologically recent dune limestone of that district. 



Petrological and Chemical Characters. — Very little systematic 

 petrological work, and still less of chemical work, have been done on 

 the newer basalts of Victoria. Howitt has described some of the 

 Gippsland rocks, and Gregory has examined some, chiefly from the 

 mining centres. 



The great bulk of the rocks of this period probably belong to the 

 basic series, and are correctly referred to the basalts. So few 

 analyses have been made of these rocks that our knowledge of their 

 range in chemical composition is very scanty. The few reliable 

 analyses which have been made, and more particularly the microscopic 

 examination of sections from different areas, suggest that a fairly 

 considerable range in composition exists from very basic limburgites 

 through olivine-basalts and dolerites, augite-basalts to rocks which 

 are highly felspathic, and probably some types should be referred to 

 the andesites. 



A special occurrence of a felspathoid from the ejected blocks of Lake 

 Bullenmerri, near Camperdown, has been identified by Mr. Mahony. 

 His description is now in the press, and will appear in a memoir of 



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