196 Proceedings of the Royal Society of Victoria. 



eastern buttress and along the road between the 

 last-named point and the foot of the ridge south of 

 the Hanging Rock. 



A variety of the dacites intei'mediate between the Willimi- 

 gongong and the merocrystalline types occurs at the southern 

 end of the Upper Macedon Spur. It is exposed on the slope 

 between Mr. H. R. Hogg's house, Cheniston, and the Willi- 

 migongong. The rock is holocrystalline and porphyritic with 

 phenocrysts of plagioclase and hypersthene, and some quartz. 

 The base under a low power in ordinary light appears glassy 

 with an incipient fluidal structure due to the banded distribution 

 of minute greenish microliths. The largest of these microliths 

 are hypersthene ; the remainder are probably also hypersthene, 

 but they are too small to show pleochroism or admit of certain 

 identification. The rest of the base is doubly refracting and 

 consists of a felt of plagioclase laths with granules of quartz and 

 plagioclase. 



The main difference between the Cheniston and the Willimi- 

 gongong types is that the former shows a passage from a 

 granulitic to a pilotaxitic structure. 



A second representative of this type is the rock (38) at 

 Cherokee's, near the junction with the grano-diorites. In this 

 case an incipient fluidal structure is due to the flakes of biotite. 

 This mineral also occurs in radial tufts around the ilmenite and 

 is more abundant than in most of the Macedon rocks. The base 

 is granulitic, passing in patches to pilotaxitic. 



2. The Cheniston Type. 



Associated with the granulitic Willimigongong dacites is a 

 rock that weathers light brown and greyish, and then appears 

 tuff"-like, owing to the occuri-ence of angular felspar fragments 

 in a light earthy base. 



As an example of this rock may be quoted that exposed on 

 the roadside by the entrance to the carriage drive at Cheniston. 



When examined microscopically this rock is seen to have a 

 hyalopilitic structure. The phenocrysts are irregular aggregates 

 of bytownite granules and of corroded isolated crystals of the 

 same material. The hypersthene has been altered and is stained 



