142 GREAT SERPENTINE BELT OF NEW SOUTH WALES, iv., 



occurred first, that of the ilmenite and augite followed; large 

 prisms of plagioclase then formed, often with quite idiomorphic 

 extremities; their outer portions are clear albite or oligoclase; 

 the inner parts are often dusty and indeterminate ; though 

 generally acid, they appear at times to have a core of andesine. 

 The mesostasis between these crystals is very varied in character: 

 sometimes it is merely a quartz-mosaic of minute grains; or it 

 may be a mosaic of minute grains of quartz and an untwinned 

 felspar, which has the same refractive index as that composing 

 the outer zones of the plagioclase crystals; or it may be a micro- 

 pegmatitic intergrowth of quartz and felspar, radiating from the 

 edges and particularly from the coigns of the large felspar 

 crystals, and these micropegmatitic fringes may be narrow or 

 may stretch across the whole space between the enclosing 

 crystals. Again, the mesostasis may consist of a pericline- 

 twinned mass of felspar, wrapping round the end of the plagio- 

 clase prisms that project into the interstitial spaces. Finally, 

 there are instances of the whole interstitial space being ofcupied 

 by a few large quartz-grains; this last type of mesostasis, though 

 fairly common in the dolerites directly associated with the spilites, 

 is absent from a purely doleritic mass like that of Hanging Rock. 

 Apatite crystals are rarer in the mesostasis than in the main 

 part of the rock; this is the reverse of a feature commonly ob- 

 served in the quartz-dolerites(16). 



With decrease in the quantity of quartz, the rock-texture 

 becomes conditioned more entirely by the position of the felspar 

 prismoids. In Munro's Creek, some of the dolerites have a 

 strongly gneissic texture, owing to the parallel position of the 

 felspars. Augite is generally fairly abundant, forming about a 

 third of the rock; it is usually roughly idiomorphic. Ilmenite 

 and apatite vary greatly in amount. 



From this stage there is a passage into the ophitic texture, 

 with a decrease in the size of the pyroxene crystals. Rocks with 

 this texture are not so common as the former, and are among 

 the purely doleritic sills, but the structure is most developed in 

 connection with the passage-rocks between the dolerites and 

 spilites (see Plate xxv., fig. 2). 



