292 N. R. Jv/nner:- 



at times greater, and at other times less than that of the quartz, 

 and hence it appears to be oligoclase or oligoclase-andesine. 

 Sphene is not uncommon as irreguhir shaped dusty masses. 

 Rarely it is present as double wedge shaped crystals. A little 

 chlorite occurs replacing the hornblende; quartz is not abundant. 

 Apparently the original rock was an intermediate plutonic rock. 



Horiihlende diabase (Epidiorife). 



Section W2. Pebble in conglomerate near the Thomson River 

 copper mine, is a holocrystalline, fairly even grained rock with 

 pilotaxitic fabric. There is a tendency towards an ophitic tex- 

 ture in places, but it is never very pronounced. Plagioclase and 

 actinolite constitute almost the whole rock, but there is also a 

 little quartz, ilmenite, chlorite and zircon present. The actinolite 

 occurs usually in anhedral grains and masses. It is moderately 

 pleochroic, varying in colour from light green to almost colourless. 

 It is ophitically penetrated by the felspar in places. Most of the 

 felspar laths are either simply twinned, or untwinned. Owing 

 to their extensive sericitisation, it is frequently impossible to 

 determine their original characters. Extinction angles as high 

 as 350 from the twin planes were measured on certain felspars, 

 indicating a rather basic labradorite. Interesting outgrowths have 

 taken place around the original felspar laths. Their idiomorphic 

 outlines are generally visible as greatly altered cores, surrounded 

 by a clear outer zone of albite showing ragged boundaries. 



One interesting case, of additional material being added to a 

 former idiomorphic crystal of felspar at two different periods, was 

 visible in this section. By the first addition tlie idiomorphic 

 crystal became rectangular in outline. Both the original crystal 

 and the added material Avere now completely sericitised, and a 

 second marginal addition of pellucid felspar took place. The 

 twinning of the original felspar extends through the clear ex- 

 terior zone, showing that the addition has been in crystallographic 

 continuity with the primary felspar. The method of formation 

 of these outgrowths has been masked in the present case by thdl 

 great changes undergone by the rock. Professor Judd,i from a 

 study of similar outgrowths on felspar crystals from the Western 

 Isles of Scotland and elsewhere, came to the conclusion that the 

 majority of such additions took place Avhilst the felspars were 

 fresh and unaltered, and that in the Scotch case the outgrowths took 



1 Quarterly Journal of the Oeoloffical Society, London. 



