138 PROCEEDINGS OF SECTION C. 



a hill composed of almost white rock. This at first sight looks like 

 sandstone, especially as it shows tabular jointing, which give it a 

 stratified appearance. A close examination of the hand specimen 

 shows that it is igneous in origin, and this is confirmed by a micro- 

 scopic examination. The section shows the rock to consist mainly 

 of felspar showing trachytic structure. Bleached remnants of what 

 was probably aegirine are present, together with some magnetite. 

 This rock is a trachyte. 



About a mile to the north of the township of Carapook trachyte 

 occurs overlying the archaean gneisses and schists. The rock is 

 greenish in colour and under the microscope shows phenocrysts of 

 anorthoclase in a trachytic ground mass. ^Egirine is the principal 

 ferro-magnesian mineral. 



One of the most interesting rocks in the area occurs near Koolo- 

 mert Station. On the side of a valley near the homestead a small 

 quarry has been opened exposing a white columnar rock. The 

 columns exposed are up to six feet long, and may be of far greater 

 length. Perfect columns about four feet long and six inches in 

 diameter have been obtained from the quarry. Overlying the white 

 columnar rock is a dense black igneous rock which weathers to a 

 light colour. Mr. Dennant concluded that the columnar material 

 was only bleached igneous rock, and in the field this seems a probable 

 explanation, but a micro-section from one of the columns shows at 

 once that the rock is a sandstone, mainly composed of sand grains 

 with a felspathic matrix. 



A section of the overlying igneous rock discloses a rock type 

 different from anything else noted in the area, and it is rather 

 difficult to find a name for it. Augite, olivine and magnetite form 

 the greater part of the rock. They are idiom orphic and were the 

 first minerals to crystallise out. These minerals are set in a clear 

 colourless ground mass consisting mainly of allotriomorphic alkali 

 felspars. In some parts moderate sized areas of this colourless 

 felspar are seen, but in general it fills up the interstices between the 

 basic minerals. This acid residuum consists of at least two types 

 of felspar. Plagioclase showing lamellar twinning is present, but 

 frequently shows a border of untwinned material. Other areas show 

 the characteristics of anorthoclase, and some portions are com- 

 pletely isotropic. Under ordinary light it is impossible to separate 

 these isotropic areas from the felspar, as the refractive indices are 

 approximately the same and the cleavage of the felspar is not 

 apparent. Under crossed nicols the separation is quite definite, 

 and then on picking up a junction it is found (by means of Becke's 

 method) that the isotropic material has a slightly lower refractive 

 index than the felspar. The simplest explanation seems to be that 

 the light coloured residuum had approximately the composition of 

 alkali felspar, and that part of it crystalUsed out as such, while the 

 remainder solidified as a glass. Unless we coin a new name we 

 must call this rock an anorthoclase limburgite. Somewhat similar. 

 rocks occur in the Macedon area. 



