158 



REPORT ON THE PETROLOGY OF THE DOLERITES 



There is no trace of any porphyritic structure or sign of flow in any of the rocks submitted 

 for examination. From the granulitic structure there are gradations into the ophitic, 

 developed to some extent in a few specimens. The plagioclase, basic labradorite, is 

 in short thick tabula?, mutually interfering, but idiomorphic against the micropegmatite, 

 less commonly against the pyroxene. Both rhombic and monoclinic pyroxenes are 

 usually present. The former, bronzite, is generally rather more nearly idiomorphic. 

 A microperthitic intergrowth of monoclinic pyroxene is very frequent, usually regularly 

 oriented, and often showing single or multiple twinning on the 100 plane, giving a herring- 

 bone or zigzag pattern. The monoclinic pyroxene has a large optic axial angle, and 

 the extinction c to C is about 45°. It is slightly striated and rarely twinned. Varying 

 amounts of ilmenite and biotite are present, the former in small plates, the latter in 

 irregular flakes usually moulded on the ilmenite. A few exceedingly small prisms of 

 apatite are present. A varying amount of a very fine-grained micropegmatite occurs 

 interstitially. All the aphanitic dolerites examined were erratics from Cape Royds, 

 and nearly all were quite fresh. Plate I, Fig. 6, is very typical of these rocks. 



The chemical composition of South Victoria Land dolerites will be seen from the table 

 on p. 157. 



On calculation the first two analyses give the following " norms " : 



I II 



100-58 



99-91 



Both of these fall into the division III, 1.4.4.3. of the American classification, being 

 slightly more acid than the rock analysed by Dr. Prior, which is an Auvergnose III, 1 .5.4.3. 



The study of these rocks may throw some light on the origin of quartz-dolerites. 

 Several hypotheses have been advanced to account for this. One would consider the 

 micropegmatite to have been derived by intrusion of granophyre into the consolidating 

 dolerite, urging in support the frequent association of soda-aplites or granophyres with 

 quartz-gabbros or dolerites. A second considers the rocks to be produced by a mixture 

 in varying proportions of a granophyric with a gabbroid magma. A third would derive 

 the granophyre from the gabbroid or magma by differentiation through fractional crystal- 

 lisation ; the acid residuum either forms the granophyric mesostasis in the rock or is 

 extruded from it to form the dykes of granophyre. Dr. Prior points out that the general 

 variation of the grain-size of the micropegmatite with that of the including rock is the 

 fact used by Dr. Holland in support of his application of the third hypothesis to the 

 explanation of the micropegmatite in the Indian " augite-diorites." * The rarity or 

 absence of veins and dykes of soda-aplite or granophyre in the dolerite masses of South 

 Victoria Land may also be taken to show that their presence is not essential to the formation 

 of quartz-dolerites. But some rocks occur, like certain collected on the Dry Valley 

 by Ferrar, and by the Magnetic Pole party in 1908-9, in which the acid material is very 



* G. T. Prior, loc. cit. supra, p. 



136. 



