352 C. A. SUSSMILCH AND W. G. STONE. 



seem hardly probable, under the circumstances, that the 

 two were intruded simultaneously; one would expect the 

 belt of slates to have been broken up and shattered under 

 such conditions. So that it seems probable that one of 

 these rocks was intruded later than the other; as to which 

 was the later there is at present no evidence to show; it 

 is not considered, however, that any great period of time 

 elapsed between the two intrusions, but that they both 

 belong to the one epoch. 



The Kanimbla Valley bathylith intrudes Upper Devonian 

 as well as Silurian strata, and the writer 1 has previously 

 assigned a late Devonian age to it (Kanimbla Epoch). The 

 quartz porphyrites are probably of the same age. 



The contacts of the intrusions with the rocks they intrude 

 are very interesting from the point of view of the methods 

 by which the intrusion took place. That there has been 

 some marginal assimilation there can be no doubt, but this 

 has been relatively small in amount and has not produced 

 any marked difference in that part of the porphyrite in 

 contact with the slate, as compared with that some dis- 

 tance away. The features of the contact strongly suggest 

 mechanical methods of intrusion, such as the theory of 

 "overhand stoping" suggested by Reginald Daly. Large 

 blocks of slate, many feet in diameter, can be seen already 

 "undercut" and apparently just ready to break away; other 

 large masses of slate twenty to fifty feet, or even more, in 

 diameter, occur entirely surrounded by igneous rock. Such 

 masses have frequently broken away along the bedding 

 planes leaving the igneous rock in places with a contact 

 like that of a sill. These features are well shown in the 

 road cuttings of the Mount Victoria road. 



1 Introduction to the Geology of New South Wales. Angus and 

 Kobertson, Sydney, 1914, p. 80. 



