658 ^ DJt. J. A. THOMSON ON THE [DeC. I9I3, 



But, as the potash is probably present in sericite, and controls 

 more alumina in this mineral than in orthoclase, and as the 

 hornblende is presumabl}- aluminous, the amount of alumina avail- 

 able for anorthite is probably not so great as the above calculation 

 assumes, in which case the preponderance of the albite molecule 

 will be more marked. Jn view of the variations shown b}* the 

 porphyrites, and their interest as a hitherto unknown member of 

 the spilite suite, the preparation of a larger series of analyses 

 may be recommended as a task well worthy of the attention of the 

 officers of the Geological Survey. 



(k) The Albite-Porphyries. 



These rocks occur in distinct dykes running approximately 

 north-north-west and south-south-east through the quartz-dolerites 

 and peridotites : they also, in close association with graphitic schists, 

 occupy a considerable area of low country between ' The Mile ' 

 and Hannan's Lake, between the south-eastern and south-western 

 dykes of quartz-dolerite. In hand-specimens they are white or 

 pink : the white varieties being coarsely porphyritie and frequently 

 deformed by shearing, while the pink varieties are less conspicuously 

 porphyritie and with difficulty distinguished from some of the 

 finer bleached quartz-dolerite greenstones. The distinction from 

 the latter can usually be made, because of the almost universal 

 occurrence of small apple-green inclusions in the pink porphyries. 



Under the microscope, the porphyritie structure is well expressed 

 by the presence of large euhedral crystals of albite, essentially similar 

 to those found in the porphyrites. There is, however, a gradual 

 diminution in the size of the albite-crystals, so that it becomes 

 difficult to determine where phenocrysts end and groundmass begins; 

 but the smallest crystals of the latter are allotriomorphic. The 

 ground-mass is much obscured by secondary sericite, carbonates, 

 and quartz: there is no suggestion, however, of orthophyric or 

 trachytic structures. 



The only evidence of any other original mineral is furnished by 

 well-defined, often hexagonal areas of chlorite, carbonates, and some- 

 times fuchsite, surrounded by a rim of minute prisms of rutile. 

 These are best developed in the strong dyke that runs from the 

 Golden Gate Station through the Hainault, South Kalgurli, Per- 

 severance, and Associated Mines, but are practically absent in the 

 dyke associated with the graphitic schists in the Great Boulder Mine. 

 They appear to be pseudomorphs of hornblende-phenocrysts ; and, 

 if this be the case, the resemblance between the porphyries and 

 the porphyrites becomes still more marked. 



The above description applies to the freshest specimens studied ; 

 frequently the rocks are extensively carbonated and sericitized, and 

 the sheared specimens are practically quartz-sericite schists with 

 augen of albite. 



Three analyses of these rocks have been published by Mr. E. S. 



