Geology of Mount Macedon. 207 



The Macedon rocks were therefore called traps, because they 

 were regarded as formino; a palaeozoic plutonic massif ; although 

 it was recognised that their aspect was not typically plutonic. 



The theory that Mount Macedon is a plutonic massif is dis- 

 proved by the absence of contact metamorphism and of disturb- 

 ance in the adjacent Ordovician rocks. There are traces of 

 contact alteration in the district ; but they may be due to the 

 intrusion of the grano-diorites. In several places the Ordovician 

 rocks may be seen close to the junction with the Macedon series. 

 Yet there is neither contact alteration nor disturbance of strike ; 

 and both changes must have happened had so great an igneous 

 mass as Macedon been directly intruded into a series of 

 sediments. 



The relations of the geburite-dacites to the underlying Palaeo- 

 zoic platform may be illustrated by the two following cases : 



1. Upper Macedon Spur. 



The Macedon township ridge which lies between the Turritable 

 and the Willimigongong Creeks, ends to the south below a band 

 of alluvium along one of the western branches of the Saltwater 

 River. On the northern bank of this stream the Ordovicians 

 can be seen outcropping below the geburite-dacites and trachy- 

 phonolites. The sedimentary beds can be traced along the 

 southern margin of the Macedon rocks and up the courses of 

 the Willimigongong as far as Cheuiston, and for a short distance 

 along a branch of the Saltwater a little to the west of the 

 Turritable Creek. The Ordovician beds show no contact meta- 

 morphism and their strike is not disturbed. The junction of the 

 sedimentary and igneous beds along the south is from the level 

 of 1600 to 1625 feet. On the eastern side of the Macedon 

 township ridge the Ordovician beds run further to the north to 

 a height of 1940 feet in the "Knoll " near Cheniston. 



The relations of the two rock series show that a lobe of 

 geburite-dacite has flowed quietly southward from a vent to the 

 north over an irregular surface of the Ordovicians. 



This conclusion is in agreement with the evidence of other 

 parts of the Macedon margin. A mile and a half to the east of 

 the Hansing Rock is an inlier of the Ordovicians runnins along 



