part 2] or southeen etee peniis^sula. 117 



(d) Petrography. 



Xo. 45, — Under the microscope, this band shows a distinct 

 blastophitic texture, for the ophitic character is now only a ' relict ' 

 feature. The constituent minerals are plagioclase, pyroxene, both 

 monoclinic and rhombic, magnetite, hornblende and biotite, and 

 a little pyrites. 



The felspar is developed in laths penetrating the augite, and 

 shows usually albite and Carlsbad twins. A pericline lamellation 

 is also dcA^eloped. Much of the felspar is filled with a minute dust, 

 rendering it semitransparent. Under a high power these inclusions, 

 are seen to consist of minute granules which are colourless, and 

 have a refractive index greater than the enclosing felspar. They 

 are not isotropic, however, and in the positions of extinction of 

 the felspar transmit light. They are probably pyroxenic granules. 

 These inclusions may sometimes be confined to the centres of the 

 felspar-laths, the dusty centre being bordered by a clear area. In 

 some of the laths they are completely absent, and in these strings 

 of pyroxene-granules often occur. This clearing of the felspars is 

 probably related to the recrystallization which has been involved. 



The felspar is a labradorite, symmetrical extinctions to a value 

 of 33° being obtained in suitably oriented laths. Some zonary 

 structure is present. 



The augite envelopes the felspar-laths, and has the t3"pical 

 appearance of the augite present in normal dolerites, showing a 

 dust of oriented inclusions. These inclusions are rod-like in section, 

 and some appear to be oriented parallel to the optic axial plane (010), 

 others again in a plane at right angles to the prismatic cleavages. 

 They are too minute for definite determination. A distinct 

 lamination of the diallage variety is observed in this augite, and a 

 curved or rosette-like arrangement of the grains may be apparent. 



At the periphery of the augite-grains, the pyroxene is often 

 converted into a clear granulitic aggregate devoid of any dust-like 

 appearance. Such aggregates consist of granules of monoclinic 

 pyroxene with a slight greenish tint, and sometimes feebl}'' pleo- 

 chroic, as observed under a high power. In some cases, only a 

 small fraction of the original dusty pja^oxene is left as a ' relict ^ 

 mineral. The pyroxene here has been clearly recrystallized with a 

 granulitic habit, and in contact with felspar where there has been 

 an approach to a crystal outline, it is the augite not the felspar 

 that is idioblastic : a feature wanting in the unaltered dolerite. 

 There are further granulitic aggregates in which an enstatite or 

 bronzite is the dominant mineral, the granules being characterized 

 by a low double refraction and straight extinction without per- 

 ceptible pleochroism. The monoclinic pyroxene is often associated 

 in granules in these and also some ' relict ' augite. This enstatite 

 is not primary but secondary, and must have been derived from an 

 original enstatite which has been recrystallized. The hornblende 

 and biotite present are closely associated with the grains of iron- 

 ore. Both these silicates are of secondary development. The 



