GEOLOGY AND PETROGKAPHY OF THE PROSPECT INTRUSION. 467 



and all other points the shale abuts horizontally on the 

 intrusive ; and secondly that the southern portion is the 

 most massive and elevated part of the intrusion. 

 6. Age of the Intruded Mass. 

 The Prospect essexite consolidated in Upper Triassic 

 strata, hence all that can be said with confidence of its age 

 is that it is Post-Triassic. By comparison with igneous 

 rocks whose age can be determined with more certainty, 

 however, there is ground for supposing that the intrusion 

 took place in early Tertiary time. 



Examination of the Post-Triassic igneous rocks within 

 and around the edge of the Triassic basin of New South 

 Wales reveals a remarkable consanguinity, all of them 

 belonging to the alkaline series. The acid and intermediate 

 rocks are obviously alkaline, being segirine-trachytes, 

 nepheline-syenites, etc., whilst recent researches of Messrs. 

 Card and Mingaye have shown clearly that all the basalts 

 of the Blue Mountains, and the numerous basalt-dykes and 

 necks nearer Sydney, so far as examined, have a distinct 

 alkaline character. 1 There is reason to believe that all 

 these eruptive masses owe their origin to the same period 

 of igneous activity, probably connected with the upheaval 

 of the Eastern Cordillera. It is necessary to consider, 

 however, whether there may not be evidence that there 

 was more than one period of activity. 



Mr. E. O. Andrews, in his papers treating of the Tertiary 

 history of the New England Plateau (N.S.W.J, finds that 

 the great basalt outpourings in that district fall into two 

 periods. He also mentions the probability of two periods 

 of basaltic eruptions in the Blue Mountain district, sepa- 

 rated by a considerable interval of denudation. 3 



1 Eec. Geo!. Surv. N.S.W., vn, pt. 3, pp. 226 and 236. 



2 Rec. Geol. Surv. N.S.W., vn, pt. 3, (1903) p. 197, and pt. 4 (1904) 

 p. 293. 



3 Loc. tit., p. 216. 



