by h. i. jensen. 907 



Discussion of the Analyses. 



The chemical examination of tlie Nandewar rocks was con- 

 ducted in the Chemical Department, Sydne}^ University, and I 

 am indebted to Professor Liversidge and Mr. Schofield for having 

 placed apparatus at my disposal. 



The alkaline rocks analysed all belong to the intermediate 

 group. Yet there are both acidic and basic alkaline rocks in the 

 district which were not analysed, having been satisfactorily 

 determined microscopically. 



Three of the rocks analysed, namely, dolerite from Dingo 

 Creek (N.17), lamprophyre. Dingo Creek (N.18), and perlitic 

 pitchstone, Boggabri (N.62), exhibit no definite relationship 

 with the alkaline series. 



The lamprophyric rock probably has a monchiquitic base. 

 Felspar is very rare; most of the AlgOg exists in the spinel 

 (pleonaste). The alkali not having sufficient AlgOg to form 

 felspar, has gone to form analcite. This rock might indeed be 

 best regarded as the result of a kind of magmatic mixture in 

 which an alkaline magma has burst through a very basic mass, 

 and carried along in it fragments of peridotite (xenoliths) and 

 xenocrysts of spinel and hypersthene. The occurrence is strictly 

 analogous to that which has been described by Mr. C. Siissmilch, 

 F.G.S., for the Bombo Quarries near Kiama.*' 



In both instances, too, we have an alkaline rock (at Kiama, 

 an orthoclase basaltf) carrying these basic xenoliths. Mr. C. 

 Siissmilch has kindly shown me his specimens, and the resem- 

 blance to mine is very striking. A similar occurrence has been 

 described at the Pennant Hills Quarry. J 



The chemical analysis of the Boggabri pitchstone gives no clue 

 as to whether this rock is of the same age as the trachytes or 

 not, a matter for which I had insufficient field-evidence to decide. 



* "On the Occurrence of Inclusions of Basic Plutonic Rocks in a Dyke 

 near Kiama." Journ. Proc. Roy. Soc. N.S.Wales, Vol.xxxix. 



+ "Geology of the Kiama-.Tamberoo Districts." Rec. Geol. Surv. N. . 

 Wales, Vol.viii. 



+ Journ. Proc. Roy. Soc. N. S. Wales, 1893. 



