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THE GEOLOGY AND PETROLOGY OF THE GREAT 

 SERPENTINE-BELT OF NEW SOUTH WALES. 



Part iv. The Dolerites, Spilites, and Keratophyres of the 

 NuNDLE District. 



By W. N. Benson, B.Sc, B.A., F.G.S., Linnean Macleay 

 Fellow of the Society in Geology. 



(Plates xxv.-xxvii., and six text-figs.) 

 Introduction. — The first three Parts of this series recently 

 issued(l) contain some of the results of field-observations made 

 during the years 1909-1911, and of petrological observations 

 made in Cambridge, whither the writer proceeded, having been 

 awarded a Research Scholarship, by the Royal Commissioners of 

 the Exhibition of 1851. Attention was devoted in the field 

 chiefly to the occurrence of serpentine, and the intricate relation- 

 ships of the dolerites and spilites were less studied, for the 

 peculiar interest attaching to them was then unknown. A 

 perusal of Messrs. Dewey and Flett's paper on British Pillow 

 Lavas(2) showed the importance of these rocks, but the material 

 that had been collected was not sufficient to allow of a detailed 

 discussion, and a re-examination of the field-evidence was neces- 

 sary. The following is the result of six weeks' further work in 

 the Nundle district, and the study of about one hundred and 

 seventy thin slices of the rocks collected. 



In the previous papers it was recognised that the coarse- 

 grained dolerites were intrusive sill-like bodies, and it was 

 believed that the fine-grained, or aphanitic, and frequently amyg- 

 daloidal spilitic rocks were lava-flows. At the same time it was 

 remarked that many passage-rocks existed, and that it was 

 frequently found difficult to refer an amygdaloidal dolerite of 

 medium grainsize either to the one group or to the other. The 

 same doubt as to the distinction of flow from sill has arisen in 

 most localities in which these rocks occur. A second difficulty 



1.3 



