S5 



dominate?, but there is much serpentinised enstatite. The 

 diallage is present in too great quantity for the rock to 

 be a norite, but the rhombic pyroxene stamps it as a 

 passage rock between the gabbros proper and norites. The 

 felspar is not plentiful, and from its high extinction angles 

 may be assigned to the anorthite group. Some of the crystals 

 of felspar show incipient decomposition in the form of opaque 

 dusty patches, which is probably the commencement of 

 saussuritisation. The titaniferous iron is granular, and in the 

 state of leucoxene. No olivine is visible. 



11. Porphyritic Diabase. 



Min. Const. : Augite, plagioclase felspar, magnetite, and 

 secondary chlorite. 



Sp. Gr. 2-9. 



Macroscopical Structure. — A fine grained, compact rock, 

 presenting a uniform dark grey colour throughout, with the 

 exception of the porphyritic crystals of triclinic felspar, which 

 are milk "white aud shining on the principal cleavages. The 

 numerous porphyritic felspars form the most conspicuous 

 feature of this peculiar diabase. They occur semi-distant 

 and irregularly embedded ; in size they vary in a marked 

 degree, the majority probably averaging about 8 millimetres 

 in length, with an occasional specimen of comparatively large 

 size. Under the hammer this rock is exceedingly tough, 

 breakiug with an irregular sharp-edged fracture. It was 

 observed in the form of loose blocks on the Waratah-Corinna 

 road, near the bridge which spans the Arthur River. As it 

 was not obtained in situ it is uncertain whether it is effusive 

 or a penetrating mass. 



Microscopical Structure. — Its main feature is the marked 

 development of the diabasic or ophitic structure, the large 

 shapeless masses of augite looking as if cut up by narrow 

 felspar prisms in all directions. The orientation, however, 

 remains the same, showing the pyroxenic substance to be one 

 and the same crystalline mass. This division of the augite 

 is essentially an optical effect ; in reality, the felspars had 

 crystallised out in their present form before the augite had 

 consolidated. When it did consolidate, it enwrapped the 

 felspars, which then remained fixed in different positions. 

 The latter is now pale green in tint, and all of it is in a 

 chloritised state. 



Another noticeable point is the presence of a few large 

 crystals of plagioclase felspar, imparting a porphyritic appear- 

 ance to the rock. This is what Rosenbusch calls the diabase- 

 porphyrite facies. It is often connected with the peripheral 

 parts of diabase intrusions. Notwithstanding this, the rock 



