VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 



135 



along some of the more pronounced cleavage planes. Among 

 these and otlier peridotitic rocks the crystallisation of free 

 picotite has been studied carefully, and it is found that no 

 constant period of solidification is indicated. 



Fig. 2. Crystallisation of Spinel in Peridotites. 



(a) Late Crystallisation. S picotite, E enstatite, B bowlingite pseudo- 



niorphs after olivine, Au augite. 

 (6) Early Crystallisation of picotite S, in association with E enstatite, 



Au augite and O altered olivine. 



Fig. 2 (a) shows the evidence for referring the formation 

 of spinel to a period late in the crystallisation history of 

 the rock. There as elsewhere it is found filling inter- 

 granular spaces and enclosing olivine. However, in many 

 cases picotite has undoubtedly crystallised first, as shown 

 in figure 2 (b). Often, too, corrosion of the spinel is seen 

 and a pretty example occurs of a small reaction rim 

 between picotite and enstatite which is unmistakably com- 

 posed of serpentine, a result which is hard to explain 

 chemically. 



The peridotites contain excellent examples of cracks 

 developed in unaltered minerals, particularly pyroxene, by 

 pressure exerted upon them by the expansion of adjacent 



