306 PETROLOGICAL NOTES_, i., 



The question of the mode of origin of these rocks is one on 

 which little can be said. From internal evidence, the writer was 

 disposed to think them all to be volcanic, but Professor David 

 states, "recently I have examined further sections in the neigh- 

 bourhood of Wallarobba (between Martin's Creek and Dungog), 

 and found, close to the Wallarobba Station, a most interesting 

 section showing Martin's Creek Andesite strongly intrusive, as 

 a sill, into the Carboniferous sedimentary rocks. Since seeing 

 this section, I have now no doubt whatever that, at Martin's 



Creek also, the rock is strongly intrusive There can be 



no doubt, I think, that sill-structure is much more extensively 

 developed in the Carboniferous rocks than we had previously 

 any idea of."* 



Mr. W. N. Benson also writes to me, concerning the hypo- 

 crystalline hypersthene-andesite of Duri Peak, near Curra- 

 bubula, "I believe it to be part of a great sill of several miles in 

 extent, and several hundred feet in width, but I have not 

 finished field-work thereon."! 



In the face of this evidence, it is unsafe to make any assertions 

 based on microscopical information, unsupported by field-rela- 

 tions. Nevertheless, the frequent occurrence of tuffs and tuffa- 

 ceous sedimentaries, renders the occurrence of lavas also in the 

 highest degree probable. 



This uncertainty of origin introduces difficulties of nomen- 

 clature. For the sake of uniformity, however, it has been 

 thought of advantage to adhere to the volcanic names hitherto 

 used for these rocks. Greater precision can be attained only 

 with increased knowledge of field-relations. 



The rocks are all subalkalic in character. 



The proportion of ferromagnesian to salic minerals in the 

 phenocrysts is small, except in (113): and where devitrification 

 has revealed the nature of the groundmass, the proportion in the 

 whole rock is seen to be much smaller. This poverty in femic 

 material has been noticed in the Pokolbin rocks (Browne and 



* Letter to author, February 23rd, 1915. 

 t Letter dated March 26th, 1915. 



