164 Ernest W. Skeats : Age of the Alkali Rocks. 



of the view that the alkali rocks of the Port (Jygnet and associate':! 

 areas belong to the Kainozoic period. Objection may be raised that 

 some of the Port Cygnet rocks are very decomposed, and a claim 

 to greater age for them may be made on that account. While, hoAv- 

 ever, some of the rocks are considerably altered so far as the surface 

 outcrops are concerned, which alone are available for examination, 

 fresh material from some of the rock types clearly related to the 

 decomposed rocks can be obtained, and in any case arguments 

 based on relative surface decomposition carry little weight when it 

 is remembered that highly alkalic rocks are more susceptible gene- 

 rally of ready decomposition, and it is usually types richest in the- 

 alkalies which show the greatest change. 



Conclusions. 



The evidence presented in this paper warrants, in my belief, a 

 revision of the previous view that the alkali rocks of the Port Cygnet 

 district are of pre-diabase age, and probably belong to the top of 

 the Permo-Carboniferous series, and it is here considered to be 

 highly probable, if not definitely proved, that the alkali rocks are 

 of Kainozoic age. The most powerful argument adduced is the 

 field evidence of the intrusion of a dyke of alkali porphyry into the 

 diabase (probably Cretaceous in age) at Little Oyster Cove, Ketter- 

 ir'g. Secondly, the close similarity of the Kettering rock with those 

 of Woodbridge and Petchey's Bay is advanced, and the general 

 similarity with some of the Port Cygnet rocks is pointed out. This 

 leads to the second and more general conclusion, that between all 

 the alkali rocks of the area consanguinity exists, arguing intrusion 

 during a single period of igneous activity. A subsidiary argument,, 

 to reinforce the view of the Kainozoic age of the series, consists in 

 a consideration of the age and nature of the alkali rocks of Aus- 

 tralasia generally, wherein it is shown that such rocks as are 

 fairly comparable in composition with the Port Cygnet series, and 

 whose age is susceptible of anything approaching precise deter- 

 mination, have been shown to belong to the Kainozoic period. On 

 these three grounds it is claimed that the alkali-porphyry of 

 Kettering in particular and the alkali rocks of the district in 

 general can be referred to the Kainozoic period with a high degree 

 of probability. 



