BY W. N. BENSON. 123 



strike-faulting took place, which has greatly disturbed the strati- 

 graphical succession, and this revision of the area makes it appear 

 probable that some modification will have to be made in the 

 detailed succession previously announced. It is hoped to discuss 

 this in a later communication, after comparative work has been 

 done in less complex areas. The consideration of the tuifs and 

 breccias (which, doubtless, are cognate with the dolerites and 

 spilites) is also reserved for future study. 



Detailed examination of the lines of contact between the 

 igneous and sedimentary rocks, shows that the extent of true 

 lava-flows has been overestimated. In nearly every instance, 

 the igneous rock is intrusive into the sedimentary rock, whether 

 it be a coarse-grained dolerite or a fine-grained spilite : indeed, 

 there has only one instance been observed where doubt can exist 

 on this point. An interesting fact brought to light in this 

 revision, is the frequent occurrence of the pillow or ellipsoidal 

 structure, which is so common a feature of British and German 

 spilite-lavas. But, though it has been held by some writers that 

 this structure is characteristic of lavas that have flowed over the 

 surface of the sea-bottom, it does not appear to be confined to 

 these. Pillow-structure is well developed in the Nundle district 

 in rocks which show intrusive contacts with the surrounding 

 sediments (radiolarian claystones), and the alternative view held 

 by other writers, e.^., TealKS) and Geikie(4)* that pillow-structure 

 may also be produced in lavas intrusive into loosely compacted 

 clays on the sea-floor, is the one most applicable to the features 

 seen in the Nundle district. The various explanations that have 

 been offered for the explanation of pillow-structure have been dis- 

 cussed by Clements{5), Daly(6), Sundius(7), Van Hise and Leith(37), 

 whose papers give extensive bibliographies of the subject. f Tem- 

 pest Anderson describes the formation of pillow-structure in recent 



* " Basic lavas flowing into water or watery silt." 

 t Since the above was written, Wilson's discussion of the origin of 

 pillow-structure in the Archaean rocks of North-western Ontario has come 

 to hand (The Geology of the Kewagama Lake Map-Area, Quebec. Geol. 

 Survey of Canada. Memoir 39, p. 50). He cites, though he disputes, 

 Lawson's statement that the ellipsoidal rocks of California are intrusive 

 (Mining and Scientific Press, No. 119, Vol. iv., 1912, p. 199). 



