ALKALI ROCKS OF DUN DAS, VIC. 137 



had been banked up against the dyke by landshp. On our recent 

 visit to the district a fresh " cut-away " across the junction of the 

 two rocks confirmed Ferguson's observations. Further, Mr. F. 

 Shilton pointed out that across the creek in a continuation of the 

 line of the dyke boulders of the trachyte could be picked up, so 

 that it appears certain that the dyke crosses the valley of the 

 Konong Wootong Creek and is intrusive into the glacial con- 

 glomerate on the farther side of the creek. 



At the back of the Konong Wootong Creek homestead more 

 trachytic rock occurs. On the eastern face of the hill at the back 

 of the house is a fine grained whitish rock, which on sectioning 

 shows typical trachytic structure, but without any phenocrysts. 



Northward from Adam and Eve, but on the opposite side of 

 the stream, is a somewhat different type. The rock is coarser in 

 grain and the felspars are distinctly visible in the hand specimen. 

 Under the microscope the ground-mass is seen to be distinctly 

 trachytic, and the rock shows evidence of flow structure, all the 

 felspars, including the phenocrysts, being approximately parallel. 

 The principal ferro-magnesian constituent is aegirine showing pleo- 

 chroism and practically straight extinction. Occasional large flakes 

 of brown biotite are present. Calcite and oxides of iron occur as 

 secondary mineral. The phenocrysts are numerous and are mainly 

 soda-sanidine, but anorthoclase is also present. 



Still further northward at Wotong Vale is a fine volcanic plug, 

 " The Giant Rock." This is situated in the valley of a tributary 

 of the Konong Wootong Creek. The Giant Rock bears a striking 

 resemblance to the Hanging Rock at Woodend, but is on a somewhat 

 smaller scale. Mr. Dennant stated that this rock was little more 

 than a mass of kaolin, but some moderately fresh material was 

 obtained. A section from this showed anorthophyric ground-mass 

 merging in part towards trachytic. The felspars are comparatively 

 fresh, but the ferro-magnesian minerals are not identifiable. Pheno- 

 crysts of anorthoclase are sparingly present. An analysis quoted by 

 Mr. Dennant of " The Little Rock," which adjoins the Giant Rock, 

 shows soda in excess over potash. 



The relationship of the rocks at Mt. Koroite is somewhat com- 

 plex. Several types of igneous rock occur, ranging from sub-acid 

 to basic. Mt. Koroite is flat-topped and is best described as a hill 

 of circumdenudation. The upper part of the hill consists mainly 

 of basaltic rocks. There is some variation in the microscopic 

 characters of different samples, but in general they may be described 

 as ohvine anorthoclase basalts. Augite and plagioclase showing 

 ophitic structure, ohvine. altered brown glass and magnetite are the 

 principal constituents. Secondary calcite is abundant in some 

 sections. Phenocrysts of anorthoclase do occur, but are rather rare. 

 In the ground-mass there are two felspars, one a plagioclase showing 

 lamellar twining and a second showing lower refractive index and 

 extremely wavy extinction. This latter is probably anorthoclase. 

 Just to the west of Mt. Koroite, but separated from it by a gully, is 



