332 Ernest W. Skeats : 



fra.gments in the Silurian conglomerates constitutes the best 

 evidence that the cherts are pre-Silurian. The total absence of 

 conglomerates at the junction of the cherts and Ordovician 

 shales is strong negative evidence that there is no strati- 

 graphical break between them, that is, that the cherts are not 

 prei-Ordovician. The field evidence strongly points to the 

 black cherts representing highly silicified Ordovician tuffs or 

 fine-grained bedded fragmental rocks made up of diabasic 

 material. The mineralogical a.nd petrological similarity of the 

 black cherts from near Gate 47, a.nd from the Lady's Pass, is 

 so close that there can be little doubt that they also aa'e altered 

 Ordovician rocks, although they do not come into relation at 

 the surface with normal Ordovician rocks. 



Specimens of banded black chert (Nobby's chert) obtained by 

 me when visiting Newcastle, N.S.W., in company with Prof. 

 David, are almost indistinguishable from some of the Heathcote 

 cherts. It is interesting to find that Prof. David^ has obtained 

 clear evidence in the field, and Mr. Card in microscopic sections, 

 that this chert is a silicified bedded tufi". Its very close resem- 

 blance to some of the Heathcote cherts suggests, therefoi'e, a 

 similarity of origin for the latter. 



Origin of the Silicification. — Dr. Howitt has claimed that 

 the silicification of the Ordovioiansi represents the direct effects 

 of contact metamorphism due to the intrusion of the diabase 

 into the Ordovician. Prof. Gregorj^ on the other hand, main- 

 tains that the cherts represent the oldest rocks in the district, 

 and were formed in pre^Ordovician times, and that the diabase 

 was subsequently intruded into them. According to him, then, 

 the cherts are not in any way genetically related to the diabase, 

 but he does not discuss at all the question of how they were 

 formed. 



Dr. Howitt has shrnvn that in some places as you recede from 

 the diabase the silicification is less intense. The change is. 

 however, not always so gradual. 



I am unable to agi'ee wnth Dr. Howitt that the silicification 

 is due to contact metamorphism by the intrusion of diabase. 

 The change does not consist in a production of new minerals by 



1 "The Geology of the Ihinter Kivev Coal Measures." Memoirs of Geol. Surv. of 

 N.S.W., Geology, No. 4, Part I., p. 17. 



