670 GREAT SERPENTINE BELT OF NEW SOUTH WALES, iii., 



Chromite is not common in the pyroxenites, but, in the perido- 

 tites, it may increase in amount, until it becomes the dominant 

 constituent. 



Associated with the peridotites and pyroxenites, are rarely 

 amphibolites and olivine-gabbros, more frequently eucrites and 

 anorthosites. No picrites, troctolites, and norites have been 

 found yet. 



The rocks will be described under the following divisions : — 

 (a) Peridotites; (b) Pyroxenites; (c) Amphibolites. 



(a). The peridotites are almost entirely harzbergites. It is rare 

 that diallage is present in sufficient amount to cause the rocks to 

 pass into the lherzolites, while the proportion of rhombic pyroxene 

 is almost always too great to allow the rock to be classed as a 

 dunite. There are three main structures developed, the granular 

 porphyritic, and poikilitic. In the first, the grains of olivine and 

 enstatite are roughly equal in size, being about 2 mm. in diameter, 

 while the small chromite-grains rarely exceed J mm. in diameter. 

 This last mineral has two forms of occurrence. In most cases, it 

 forms irregularly shaped, but not angular grains. In other cases, 

 it is quite granophyric in habit, running in irregular, twisting 

 and branching strings; and while not forming a definite grano- 

 phyric intergrowth with its host, it seems generally associated 

 with monoclinic pyroxene. Sometimes the strings of chromite 

 rise perpendicular from the outer boundary of the enclosing 

 crystal [bastite in N.T., 238]. 



In the porphyritic rocks, the pyroxenes are distinctly larger in 

 size than the olivine. As is seen elsewhere, in thoroughly serpen- 

 tinised rocks, there may be developed a false porphyritic appear- 

 ance, owing to the enstatite changing intact to large plates of 

 bastite, while the olivine has formed a small mesh work of serpen- 

 tine. In the poikilitic or lustre-mottled types, large bronzite 

 crystals form the ground-mass of the rock, in which smaller 

 olivine-grains are set. The best examples of this type of rock 

 may be obtained on Chrome Hill, behind Bowling Alley Point, 

 where plates of bastite four inches long, studded with serpentinised 

 olivine, have been collected. They are a deep brown in colour. 



