136 PROCEEDINGS OF SECTION C. 



The Alkali Rocks. — Under this heading it is proposed to give a 

 description of the occurrences and characters of the alkali rocks. 



Lying to the north of Coleraine, and ]ust separated from the 

 township by the Koroite Creek, is a ridge composed of trachytic 

 rocks. At the south-western end of the hill a deep quarry exposes 

 a good section. Near the surface the trachyte shows signs of con- 

 siderable alteration, and is light-brown to whitish in colour. In 

 the deeper parts of the quarry, however, the rock is comparatively 

 fresh and is greenish-brown in colour. This rock has been described 

 by Professor Skeats^ as an anorthoclase trachyte. The felspars are 

 mainly anorthoclase, but some soda-sanidine is also present. 

 iEgirine and segirine-augite are the principal ferro-magnesian 

 minerals, but both are altered. Chlorite and aragonite occur as 

 secondary minerals. Near the Konong Wootong Creek estate 

 are situated the hills Adam and Eve, a third hill in the same line 

 being known locally as The Devil. These three hillocks occur on an 

 east and west line and are all connected by a ridge with one another. 

 The general appearance suggests a fissure eruption, the hillocks 

 being formed by local widening of the fissure. 



Mr. Hogg has described the basic rock of Adam as an olivine 

 basalt ; but it differs from normal basalt in that corroded crystals 

 of anorthoclase are not infrequent. The ground mass is fine grained 

 and consists of plagioclase (andesine to labradorite) and a slightly 

 pleochroic augite with abundant magnetite. Numerous pheno- 

 crysts of olivine and some of augite, the former showing incipient 

 alteration to serpentine, are, together with anorthoclase, the porphy- 

 ritic constituents. The anorthoclase is similar to that which occur 

 in the alkali rocks of the Macedon area. Numerous spherulitic 

 aggregates of small crystals of augite are scattered through the 

 section. This rock should be described as an anorthoclase olivine 

 basalt. 



Running east and west through these hills is a light-coloured 

 trachytic dyke. This dyke has been fairly extensively quarried for 

 building stone, the Konong N^^ootong Creek homestead being built 

 of this material, and the toning in the stones due to varying amounts 

 of iron is very effective. The ground mass is holocrystalline and has 

 the typical orthophjn^-ic structure, the felspars being short, stout 

 prisms of anorthoclase. Throughout the ground mass is consider- 

 able oxide of iron, mainly the hydrated form, and this is undoubtedly 

 due to the alteration of some ferro-magnesian mineral, probably 

 gegirine. Original magnetite is rare. The phenocrysts are fairly 

 numerous and consist entirely of felspars. In some cases there is 

 a central core of plagioclase surrounded by a fringe of alkali felspar, 

 Soda-sanidine and anorthoclase are both present, the latter showing 

 at times almost ultramicroscopic twining. The anorthoclase also 

 shows a perthitic intergrowth of albite. Ferguson pointed out that 

 this dyke was not only intrusive into the basic rock but also into the 

 glacial conglomerate, but Hogg considered that the conglomerate 



1 N/j. cit. p. 207. 



