﻿280 THE ORDOYICIAST OF THE GLENSAUL DISTRICT. 'May I9IO, 



described by the Authors, he was much interested in their descrip- 

 tion of the association of cherts with tuffs and igneous rocks in the 

 Arenig Series, and in their demonstration that some of the cherts 

 were silicified tuffs. A similar association of diabase, tuffs, and 

 cherts occurred at Heathcote and other areas in Victoria. Many 

 of the banded cherts from Heathcote could not be distinguished 

 from the cherts exhibited by the Authors. 



Mr. Gardiner expressed the thanks of the Authors for the re- 

 ception accorded to their paper. In reply to Mr. Thomas, he pointed 

 out that, although the lowest graptolitic horizon was undoubtedly 

 referable to the Didymograptus-extensus Zone, the highest one con- 

 tained only a few graptolites — from which no exact conclusion 

 could be drawn. The Authors, therefore, merely put forward the 

 suggestion that these beds might belong, either to the D.-hirundo 

 Zone, or to higher beds. 



"With regard to the reason why the felsite sill formed a hill-crest 

 at one end and a valley-floor at the other end of the area, he 

 pointed out that these rocks had been exposed to more than one 

 period of denudation. The ?Bala conglomerate, which was full of 

 fragments of felsite and of chert, doubtless extended at one time 

 over the whole area. The trend of the present river-system might, 

 therefore, be regarded as being due to causes other than the occur- 

 rence of the felsite in particular spots. 



