374 GREAT SERPENTINE BELT OF NEW SOUTH WALES, vil., 



grouiidmass here becomes exceedingly minute in grainsize, and 

 ^e^y dark with finely divided magnetite. The groundmass is a 

 mosaic of minute interlocking grains of quartz. If felspar be 

 present, it is entirely untwinned, and the abundance of the mag- 

 netite prevents its recognition by the bright-line method. The 

 distribution of lath-shaped areas in the rock free from magnetite, 

 which w^ere doubtless originally felspar, indicates a general poorly 

 developed trachytic structure. Lai-ge areas rich in magnetite 

 appear to replace pyroxene-phenocrysts, and, in addition, there 

 are abundant phenocrysts of albite. This rock is possibly an 

 altered form of an augite-porphyrite. 



The group of the Silicified-Nodular or Blotched Keratoplinres 

 comprises a series of very remarkable rocks occurring in the 

 southern end of the eastern zone of keratophyre. In hand-speci- 

 men, they are very fine-grained or aphanitic green rocks with 

 masses of silica, weathering out into beads set irregularly or in 

 long rows as in spherulitic rliyolite, or covering the whole surface 

 of the rock with a network-pattern of cracks weathered out between 

 more resistant silicitled patches. Microscopically, the rock, in its 

 unaltered state, Avould be classed among the more basic pyroxenic 

 keratophyres. It 1ms a trachytic base of small laths of acid felspar, 

 with an abundance of chlorite pseudomorphs after granular augite, 

 and scattered small grahis of magnetite. In this base are pheno- 

 crysts of albite, sometimes arranged in groups (the glomero- 

 porphyritio structure), a few irregular masses of chlorite, which 

 possibly represent original pyroxene-phenocrysts, and some small 

 phenocrysts or aggregates of magnetite. Well defined prisms of 

 apatite often appear in the vicinity of the phenocrysts. The 

 resistant portions of the rock prove to be rounded or irregular 

 regions in which the pyroxene has been completely removed, and 

 the felspar-laths remain apparently unaltered, lying in a matrix of 

 quartz which is in optical continuity over an irregularly bounded 

 area. Each resistant region or "pseudospherulite" may consist of 

 several such areas of quartz. Where the rock is traversed by a 

 small vein of quartz cutting through one of these replacement- 

 areas, the quartz in the matrix of the rock, on either side of the 



