206 KEV. J. MILNE CURBAN. 



allotriomorphic augites. There is no ground-mass, the spaces 

 between the plagioclases being filled with augite, olivine and 

 isotropic glass. The olivine is idiomorphic, but has suffered greatly 

 from secondary alteration. The rich brown-red patches, already 

 referred to, are the result of this alteration, and the process can be 

 noted in every stage. Large brown-red masses may be seen in 

 the slice, some preserving the characteristic shape of many of the 

 olivines of basalts, that is, in boat-shaped figures that represent 

 slices either parallel to O P or to o> P oo. Other sections are to 

 be found in this slice with an as yet untouched nucleus of clear 

 olivine. The magnetite occurs in grains, and is no doubt, a 

 mineral of the first consolidation. Besides these grains, there are 

 rods of magnetite, very often immediately around the most altered 

 •olivines. These rods are most probably, of secondary origin. 

 The plagioclases are seen to have more complete terminations 

 than is usual in basalts. In polarized light, the almost non- 

 crystalline character of the rock, with the broad prisms of felspar 

 give the slice a particularly rich appearance. The augite, too, 

 polarizes in colours rather more brilliant than usual. A few of 

 the idiomorphic olivines are seen to contain cores of devitrified 

 glass. 



Slice 22. — Dolerite Basalt, eight miles north-east of Dubbo. 

 In hand specimens, this rock would, without doubt, be called a 

 dolerite. A microscopic examination alone, of some of the slices 

 would lead one to term the same rock a diabase. However, when 

 studied in the field, there is no doubt left in the mind of the 

 geologist that he is dealing with a coarse-grained and altered basalt 

 which graduates through every variety into the average basalt. 

 Macroscopically. the rock has a mottled greyish-black crystalline 

 appearance. It weathers to a light granular grey. In fractured 

 chips, it shows a faint green on the translucent thin edges. In 

 ordinary light under the microscope, it has the general appearance 

 of an ophitic dolerite, with grains and aggregations of magnetite 

 scattered through the field. The augites are idiomorphic and of 

 •a rich brownish-yellow colour. They are much fractured and 



