581 



Melrose. The general field occurrence of these rocks has 

 been described by Mr. Howchin, with whom I had the 

 pleasure of spending several days mapping in their relation- 

 ships to the surrounding rocks. 



The igneous rocks fall into three groups: — (1) Altered 

 Dolerites; (2) Quartz Porphyrites (including Quartz-cerato- 

 phyre) ; (3) Aplites. 



i. Dolerites. 



The rocks of this group are the most abundant, occurring 

 in a number of small isolated patches within a relatively 

 restricted area, as will be seen from Mr. Howchin 's map. 

 They vary considerably in texture, from a coarse-grained 

 type, approaching a gabbro, to compact examples of a basaltic 

 character. Both microscopically and in thin section they very 

 closely resemble the. basic rocks of Blinman, which have been 

 described in an earlier volume of this Society's Transactions 

 by W. N. Benson. W 



Slide No. 1. — Altered Dolerite. 



Macroscopic Characters. — This example is a medium- 

 grained holocrystaliine rock, with a prevailing greenish-grey 

 colour. Ferromagnesian minerals are dominant and the 

 structure is granular. 



Microscojnc Features. — Considerable mineral rearrange- 

 ment has taken place, but in general a definite ophitic struc- 

 ture is recognizable, though as a rule the breaking -up of the 

 original minerals has developed a granular character. The 

 ferromagnesian minerals are the most abundant, and the 

 unaltered forms are chiefly pyroxene, but uralitization has 

 proceeded to some extent. A little granular material of high 

 relief is probably olivine. 



The felspars, where definitely determinable, are triclinic, 

 giving extinction angles on the albite twin lamellae up to 25°, 

 representing probably labradorite. They generally show con- 

 siderable alteration, passing into epidote and calcite. Iron 

 oxides are present, but are not very abundant. 



Slide No. 2. — Altered Basaltic Type. 



This example is a fine-grained, compact, rock of a greenish- 

 grey colour. 



Under the microscope this section shows abundant long, 

 slender, felspar laths, set in a fine-grained devitrified base, 

 crowded with magnetite showing good cubic and octahedral 



(l) "The Basic Rocks of Blinman, South Australia, with Notes 

 on Associated or Allied Rocks," W. N. Benson, B.Sc, Trans, and 

 Proc. Roy. Soc. S. Aus., vol. xxxiii. 



