48 Diorites and Granites of Swift's Creek, 



and gneissic diorites at Lower Riley's Creek, and on the hills 

 between that place and Long Gully and the hills near 

 Eureka. Diabase is rare, and so far seems only to occur at 

 the lower part of Swift's Creek in the altered schists. 



Compact Felsites. — Dykes of this class occur only, so far 

 as I have observed, at the source of the Sheep Station 

 Creek, and not far from the intrusive masses of anorthite 

 diorites. They traverse the granitic rocks at that place in 

 directions approximately north-west and south-east, the 

 dip, if any, being probably to the south-west at a high angle. 



Slices were prepared from four large parallel dykes, 

 occurring within a distance of about twenty chains, and from 

 six to twenty feet in width. The appearance of the dyke- 

 stone was in each case felsitic compact, of a greyish or bluish 

 grey tint, and seemingly much less affected by atmospheric 

 influences than the enclosing granitic rocks.* 



Three of these dykes as examined in thin slices formed a 

 series differing only in the relative minuteness of the mineral 

 components. In one the structure is so minute as to be 

 almost crypto-crystalline. In it, however, faint traces of 

 minute felspar prisms are visible, with some few porphyritic 

 crystals of the same. Here and there are small patches of 

 a dichroic viriclite, but the amount is small. 



The second sample has a crypto-crystalline, grey coloured 

 ground mass, in which are many minute plagioclase prisms, 

 and fibres and patches of viridite. In this are elongated 

 larger prisms, having the form usual in plagioclase, as also 

 some iron ore (probably magnetite). 



The third example has a micro-crystalline ground mass, 

 composed, apparently, of felsitic particles, a little quartz and 

 viridite, both diffused in fine particles, and as flakes and 

 fibres. In this ground mass are — (1) Rather long and 

 narrow interlacing or approximately parallel prisms, having 

 the habit of plagioclase, (2) minute aggregate of some 

 micaceous mineral, (3) magnetite, (4) quartz grains filling 

 in spaces, (5) much viridite in patches. 



In all these examples the felspar has been completely 

 altered to minute aggregates of some micaceous mineral ; no 

 striations remain anywhere visible. Their form, however, 

 is exactly that of plagioclase. The rock itself has evidently 

 been much altered. The viridite (chlorite ?) may be referred 



* These granitic rocks are intermediate in composition between amphibole 

 granite and quartz diorite, 



