Age of tJie Alkali Rocks. 163 



Tocks of Australasia generally. Dr. Jensen, l among others, has 

 -discussed this problem. The age of some of the alkali rocks of 

 Australasia is not yet susceptible of exact determination, since they 

 only come into relation with and intrude rocks of high antiquity. 

 Many were referred by Dr. Jensen to the Eocene period in New 

 ■South Wales and Queensland, on somewhat slender evidence. The 

 alkali rocks of New Zealand, described by Professor Marshall and 

 others, are referred generally to the middle or upper part of the 

 Kainozoic, and those of Victoria so far as known appear to belong 

 to the period immediately preceding the newer basalts; that is, to 

 the Mid-Kainozoic or the lower part of the Upper Kainozoic. 



Some years ago, in a paper on the Volcanic Rocks of Victoria,''^ 

 I tentatively included among Palaeozoic volcanic rocks certain 

 alkali rocks in Eastern \'ictoria. This reference was based on Dr. 

 Howitt's observations. Since then I have visited some of these 

 •districts in the field, and have been impressed by their recent 

 looking characters, and now believe that they are probably of 

 Middle to Upper Kainozoic age. The Mittagong-Bowral series of 

 alkali rocks in New South Wales intrudes the Triassic sediments, 

 and may well be Kainozoic in age. They are interesting in this 

 connection, since they probably come nearest in chemical and 

 mineralogical characters to the rocks of Port Cygnet, as they 

 include syenites allied to bostonite, and aegirine-arfvedsonite-quartz- 

 trachytes. The only alkali rocks in Australia definitely proved to 

 be Palaeozoic in age are the series of alkaline eruptive rocks of 

 the Cambewarra-Kiama districts, south of Sydney. These rocks 

 -consist mainly of lavas and tuffs, partly contemporaneous with the 

 Upper Marine series of the Permo-Carboniferous, and partly with 

 the Bulli coal measures. Their petrographic and chemical charac- 

 ters are, however, quite distinct from other alkali rocks in Aus- 

 tralia, so far as known, and from the rocks of Port Cygnet, as they 

 contain generally a good deal of potash, and are described as 

 Orthoclase-basalts. 



Apart from these Orthoclase-basalts of exceptional characters, it 

 will be noted that other occurrences of alkali rocks intrude various 

 members of the mesozoic, and may, therefore, be of Kainozoic age, 

 while a considerable number are definitely known, not oiily to be of 

 Kainozoic age, but to be not older than the Mid-Kainozoic. So far 

 as analogy with other Australasian occurrences go it is in favour 



1 Proc. Linn. Soc. N.S.W., 1908. 



2 Aust. AiBOC. Adv. of Sc, Briiban*, 1909. Prei. Add. to Sect. C. 



