208 Proceedings of the Royal Society of Victoria. 



the continuation of the north-eastern buttress of the main ridge. 

 The Ordovicians outcrop here as denudation has exposed the 

 summit of an old ridge of grits and sandstones. The Ordo- 

 vicians are flanked on both sides and on the south by geburite- 

 dacites ; on the sides the dacites are at a lower level than the 

 Ordovicians (Fig. 3.) At the northern end of this inlier the 

 Ordovicians are covered by a boss of basalt which forms a bare 

 hump (2200 feet high) crossed by the main road between 

 paddocks Nos. !) and 17 on the parish map of Newham. 



The Ordovician rocks are neither altered nor disturbed by 

 the dacites. 



The relations of the two series may be best explained by a 

 flow of dacites from the south having buried a meridional ridge 

 of Ordovicians. 



The evidence of these two cases shows that Mount Macedon is 

 not a plutonic massif. The second explanation that suggested 

 itself was that the mountain might be a laccolite, a view 

 supported by the apparent absence of tuffs and the general 

 form of the igneous mass. I could find no definite evidence in 

 favour of this view, and finally the discovery of beds of pyro- 

 clastic rocks ti'aversed by dykes show that this explanation was 

 unnecessary. 



This evidence leaves no doubt that Mount Macedon is the 

 worn stump of a great volcano. There is no trace of a crater, 

 and the superficial beds of ash have been removed by denudation. 

 The fragmentary material was probably never very abundant, 

 for lavas formed most of the volcanic pile. Like the phonolite 

 domes of Europe, the richly alkaline lavas of Macedon probably 

 welled forth quietly with only occasional explosions. The erup- 

 tion therefore formed a dome-shaped hill of lava with but few 

 interstratified ash beds. The first eruptions were of geburite- 

 dacite, wliich forms the main mass of the mountain. The 

 solvsbergites and trachy-phonolites were then discharged from 

 secondary vents on the flanks and probably also from the main 

 crater. The soda-andesites of the north-western flanks were 

 then erupted. Finally the Macedon eruptions ceased and the 

 volcanic forces found vent in the discharge of the basalts that 

 now cover the surrounding plains. 



