REPORT ON THE PETROLOGY OF THE DOLERITES 



157 



shown by the presence of notable amounts of biotite and orthoclase. The rock has thus 

 affinities with the essexites. It is seriate porphyroid, but distinctly dopatic. It contains 

 hypersthene, augite, and biotite in a meshwork of felspar laths, with occasionally pheno- 

 crysts of plagioclase. Orthoclase and quartz occur interstitially, and a little apatite and 

 ilmenite are present. Basic labradorite is the predominant mineral, hypersthene the 

 more abundant pyroxene, strongly pleochroic and slightly decomposed. Biotite forms 

 irregular flakes, and in one instance appears to be intergrown with hypersthene. A 

 very little brown hornblende occurs on the border of some of the hypersthene crystals ; 

 it is possibly secondary. The interstitial orthoclase is sometimes intergrown with 

 quartz. A few octahedra of magnetite are present (see Plate I, Fig. 4). 



Erratic No. 1100 is the only sample of a hypocrystalline rock submitted. It is 

 strongly ophitic, the pyroxene, the optic axial angle of which is near 90°, being very 

 much divided by the felspar mesh. Irregularly shaped areas occur made up of a dark- 

 brown glass, partly altering to chlorite, and containing idiomorphic or skeleton felspar 

 crystals and a large amount of magnetite, as skeleton crystals or dust. Some plagio- 

 clase phenocrysts are developed (see Plate I, Fig. 5). 



The aphanitic dolerites are all very fine in grain. They are dark greenish-grey in 

 colour, translucent on thin edges, and in general appearance somewhat like hornfels. 

 They are holocrystalline, with an average grain-size of about - l millimetre in diameter. 

 The structure is granulitic ; small grains of pyroxene, either irregularly equidimensional 

 or slightly prismatic, lie enclosed in a meshwork of felspar tabulae of rather smaller size. 



The Chemical Composition of Dolerites from South Victoria Land 



I. Erratic No. 1545, Cape Royds ; Porphyritic quartz-dolerite with bronzite. 



II. Erratic No. P. 366, Cape Royds ; Aphanitic quartz-dolerite. 



III. Quartz-dolerite from Knob Head Mountain ; analysed by Dr. G. T. Prior. 



IV. Enstatite-augite bearing diabase from Launceston, Tasmania ; analysed by Professor 



Dittrich. 



V. Konga-diabase from North-West Bay, Tasmania ; analysed by Dr. Pohl. 



VI. Quartz-diabase from the Cuyuni River, British Guiana. 



