206 Proceedings of the Royal Society of Victoria. 



intrusive bands of geburite-clacites. On the platform immediately 

 south of the Camel's Hump there are exposures of geburite- 

 dacitrs of the Willimigongong type, upon which rests the hum- 

 mock of the Camel's Hump solvsbergite. This rock is jointi^d 

 and weathers spheroidally. It appears to be part of a sheet that 

 flowed southward. But there is no trace of the vent, for the 

 rock is abruptly cut off by the steep northern face, which, below 

 the solvsbergite, consists of geburite-dacite. At the foot of the 

 northern face is the valley of the Five Mile Creek ; the floor is 

 covered by alluvium. Beyond this is the Hanging Rock, a hill 

 of solvsbergite, which rises 360 feet above the surrounding plain. 

 The rock is very coarsely jointed and is in places irregularly 

 columnar ; it has been worn into great hollows by the ready 

 decomposition of the soda silicates and the removal of the 

 insoluble residue by the wind. 



The Hanging Rock is probably part of a sheet discharged from 

 a vent a little to the south ; but the vent may be hidden under 

 the site of the hill. North of the Hanging Rock is a plain of 

 andesite, which in places rises into hills such as the Jim Jim or 

 Dryden's Hill. The andesite is covered in places by the basalts ; 

 and the line of demarcation between these rocks is indefinite, 

 although the country is open. The apparently intimate field 

 relations of the Macedon andesites and the basalts suggests that 

 there is no very great difference in age between them. 



It was from the supposed great antiquity of the Macedon 

 rocks that they wex'e called " traps." According to Page's^ 

 definition, the word " is now employed by geologists to embrace 

 all the multifarious igneous rocks that belong to the Palaeozoic 

 and Secondary epochs, as distinct from the more ancient granites 

 on the one hand and recent volcanic rocks on the other." 



It was apparently in this sense, and not in the original 

 meaning of a number of sheets of igneous rocks forming a 

 succession or steps {trappa, a step), that the word trap was 

 adopted by the Victorian Survey ; for the English Survey thus 

 used the term in its work in North Wales, where Selwyn 

 received so much of his training. 



1 Page, Handbook Geological Terms, 1865, p. 442. 



