121 



brown anhedrons. Titaniferous magnetite also occurs in 

 large amount among the ferromagnesian minerals, and 

 these with magnetite and apatite are concentrated into par- 

 allel bands. A few grains of titanomorphite (secondary 

 sphene) occur. The apatite is a grey-brown in colour; it 

 shows the basal parting well, the prism cleavage slightly. 

 Name. — Diopside-hornblende-diorite. 



580. Locality. — Small quarry in Cemetery Reserve on the 

 back road from Houghton to Inglewood, Portion 5657, Hun- 

 dred of Yatala. MacroscoincaUy an even-grained rock ; domi- 

 nantly felspar with diopside and ilmenite. Colour, an apple 

 green. Microscopically (see plate iii., fig. 3). — Fabric hyp- 

 idiomorphic granular, with a small amount of marginal crush- 

 ing. Grainsize fairly even, about 2 mm. The predominant 

 mineral is a basic oligoclase, the composition of which is 

 about Abj,j Augy. The grains vary in size, the largest be- 

 ing about four times the diameter of the smallest, not con- 

 sidering the finely-crushed material. While often anhedral, 

 the grains approximate to a prismatic habit. Albite twin- 

 ning only is developed. The plagioclase is but slightly kao- 

 linized. Microcline is present in small amount, included in 

 or intergrown with the plagioclase, and also occurring inter- 

 stitially and among the shattered grains. Orthoclase is also 

 present in small amount. Diopside follows plagioclase in 

 order of relative abundance. It is light-green in colour and 

 has not become uralitized to any extent. It occurs in small 

 terminated j^risms and sometimes in ragged grains. It in- 

 cludes a little magnetite primarily, and in its cracks limonite 

 may form. A very little epidote also occurs. Titaniferous 

 magnetite is present in large amount. It is very fresh, hav- 

 ing no trace of leucoxene. It is aggregated into roughly 

 lenticular patches, and these, with sphene and diopside, give 

 the rock a slightly gneissic appearance. Sphene occurs char- 

 acteristically and in considerable amount, while apatite is 

 present also in some abundance, forming rather large irre- 

 gular or hexagonal grains. The rock would be best termed 

 a diopside diorite. 



Before the examination of the rocks microscopically was 

 thoroughly complete, this rock was selected for analysis as 

 being the least altered of any of the Houghton rocks, for the 

 diopside was not uralitized and epidote was present in only 

 very small amount. It is unfortunate, however, that it should 

 also be the rock with most basic felspar, so that it represents 

 not an average composition but one unusually basic. Nearly 

 every other rock in the field would probably have a higher 

 percentage of alkalies and silica and a less amount of lime 



