BY L. A. COTTON. 831 



Quartz-dolerite. hoc. — Oakey Creek. 



Crystallinity : holocrystalline. 



Grainsize : evengrained, the typical crystals varying from 2 to 

 6 mm. 



Fabric : a network of plagioclase felspars with subordinate 

 augite. The interstitial material is a brown chlorite, with a few 

 decomposed biotite crystals. 



The chief minerals, in descending order of abundance, are 

 oligoclase, labradorite, augite, chlorite, magnetite, biotite, and 

 quartz. 



A little secondary hornblende is present, and the felspars, 

 though fairly fresh, on the whole, show some signs of kaolinisation 

 along some of the major cleavage-cracks. The felspars vary from 

 oligoclase to basic plagioclase. The most basic felspar-crystals, 

 as determined by the method of Michel Levy, are labradorites 

 of the composition Abo An 3. Many of the crystals are strongly 

 zoned, and the different zones vary widely in composition. The 

 outer edge is often oligoclase, and the central area anorthite. 

 The augite, which is very fresh, is an almost colourless variety, 

 with a faint colour suggesting that it is titanium-bearing. It is 

 intermediate in composition between diopside and true augite. 



The most interesting, and, at the same time, most puzzling 

 feature, is the presence of the brown, chloritic, interstitial 

 material, and also idiomorphic quartz-crystals The chlorite is 

 often arranged in a manner which suggests that it has been 

 derived from an augite, which was originally involved in an 

 ophitic structure with the plagioclase. The structure has been 

 variously interpreted by different observers. Professor Bonney 

 and Professor David have suggested that the chlorite is a second- 

 ary product derived from a primary hornblende. Dr. A. Thomp- 

 son has considered its derivation from augite unlikely, because 

 of the freshness of the augite associated with the felspar in the 

 rock. He has suggested that the chlorite may represent a 

 de vitrified glass. 



There is, however, a constant association of idiomorphic quartz- 

 crystals with the chlorite, which has not, so far, been recorded. 

 In Plate xci., fig. 3, this arrangement may be seen. The small, 



