REPORT ON THE PETROLOGY OF THE DOLERITES 155 



" pyroxene-perthite," occurs in a few instances in the phaneric rocks and is best studied 

 there, but it is almost always present in the aphanitic dolerites. Parallel growth of the 

 two pyroxenes without definite intergrowth is also a marked feature of some of the 

 porphyritic rocks (see Plate I, Fig. 3). 



Enstatite-augite * is the normal pyroxene in the phaneric quartz dolerites. It forms 

 large grains, ophitic to roughly prismatic in habit, and of a pale purple-grey colour. 

 It is frequently twinned on the 100 plane, usually singly only. Besides the prismatic 

 cleavages there is often a marked striation parallel to the basal plane, due partly to the 

 presence of a very fine cleavage in that direction, but also to twinning developed on 

 a very minute scale. It is only in a few favourable instances that these lamellae are 

 sufficiently thick to be clearly seen. Twinning on the 100 plane gives rise to the herring- 

 bone structure, in which the striations on one side of the twinning plane stand at an 

 obtuse angle to those on the other. The extinction angle c to C does not vary greatly, 

 being between 40° and 45°, but the optic axial angle 2E varies very much. In the 

 same slide crystals may occur, the optic axial angles of which vary from 90° to 0°. 

 The most frequent values are those lying between 0° and 30°, and from 65° to 90°. | 

 Values intermediate to these have been noted but are rare. In sections of ordinary 

 thickness there is no sign of pleochroism. 



The normal augites in which the optic axial angle is large occur in those rocks in 

 which rhombic pyroxenes are abundant, and in those that are free from micropegmatite. 

 In the latter they are usually very ophitic. 



The alteration of rhombic pyroxene commences with the development of pale-green 

 fibres parallel to the vertical axis, and growing in from the periphery, and from the 

 irregular cracks which traverse the crystal. A completely altered enstatite recalls the 

 appearance of serpentinised olivine. The alteration of enstatite-augite, and sometimes 

 of normal augite, commences with development of fibres parallel to the basal striation, 

 greenish-brown in colour, and probably chlorite. With increasing alteration a dense 

 mat of fibres is formed, sometimes with an irregular, though sharply defined, outline. 

 In apparently unstriated grains the alteration may commence in the centre and is 

 accompanied by a decrease in birefringence. Very finely divided carbonate material 

 is sometimes developed with the chlorite fibres. Sometimes brown mica is produced, 

 possibly by interaction with the felspar ; it forms small flakes and rosette-like aggregates. 

 These latter forms of alteration are those that affect the normal augite most. In the 

 pyroxenes there are frequently minute liquid-filled cavities, but it is not certain that 

 these are of secondary origin. 



Iron ore occurs in varying amount, either in irregular grains or as plates ; it is nearly 

 all ilmenite. It is nearly always present in the aphanitic dolerites, but occurs less 

 frequently in the phaneric rocks. Biotite is sometimes present in small red-brown 

 flakes. In one instance only is it present in notable amount. 



In the intergranular spaces of the rock there is frequently a micropegmatitic inter- 

 growth of quartz and felspar. The coarseness of grain of this is often proportional to 

 that of the rock itself. The felspar may be fresh or clouded, and has a lower refractive 

 index than Canada balsam ; generally it is recognisable as orthoclase. In a few instances, 

 however, it appears as apophyses from the plagioclase crystals, more acid than their 

 outermost zone, and may therefore be albite. A second type of mesostasis is sometimes 

 seen, particularly in certain rocks collected by the Magnetic Pole party. This is very 

 much finer in grain-size. It is composed of cloudy orthoclase, containing thin strips 

 of a clear colourless mineral, more strongly refractive, and disposed in parallel bundles 



* W. Wahl, " Die Enstatitaugite," Tschcrmak's Min. unci Petr. Mitt., 13d. xxvi, pp. 1-131. 

 f These figures are rough approximations only. 



