Geology of Mount Macedon. 193 



The relations of the two rocks can be best explained by the 

 Macedon rock covering the surface of an older plutonic rock. 



Petrographically there is nothing especially to connect the 

 grano-diorites and the rocks of the Macedon eruptive series. 

 Most probably the grano-diorite was intrusive into the Ordo- 

 vicians, and is simply part of the old Palaeozoic platform. The 

 grano-diorite, therefore, belongs to a much earlier period than 

 the Macedon eruptions. 



This view, however, would be refuted if Aplin's map be 

 correct as to the " granite " dyke on the western flank of the 

 mountain. This dyke is marked as running from the south- 

 eastern corner of allotment No. 22, almost due south-east toward 

 the summit of the south-western shoulder of Macedon. The dyke 

 is 2^ miles east-south-east from Woodend. In a short search for 

 this granite I failed to find any plutonic rock ; but, in what 

 appeared to me the corresponding position, is an unusually 

 coarse-grained variety of the Macedon dacites. The occurrence 

 of a granite dyke cutting through the traps would certainly 

 prove that the " granites " (in Aplin's sense of the term) were 

 later than the Macedon ei^uptive rocks. But, unless I failed to 

 find Aplin's dyke, the rock has no connexion with the grano- 

 diorites exposed on the eastern flanks of Macedon. 



2. The Rocks of Mount Macedon Series. 



The rocks of the Macedon series may be divided into five 

 groups : (1) a series of hypersthene-dacites, which for reasons 

 related on p. 202 are described as geburite-dacites ; (2) trachy- 

 phonolites ; (3) solvsbergites ; (4) alkaline-andesites ; (5) agglo- 

 merates and ash. 



a. The Geburite Dacites. 



The main bulk of Macedon is formed of a fine-grained rock 

 which, when fresh, is tough and varies in colour from dark 

 green to black speckled with grey or pinkish spots. The rock 

 weathers brown or pinkish-red. Examined microscopically the 

 rock shows two main varieties ; the first ranges in structure from 

 granulitic to pilotaxitic ; the second variety is hyalopilitic. 



