136 



G. D. OSBORNE. 



olivine grains, as a result of alteration to bowlingite and 

 serpentine; one such example, in a rock allied to wehrlite, 

 is figured in text fig. 3. Such structures are common in 

 altered ultrabasic rocks. 1 



Fig. 3. Augite in Lherzolite, showing cracks produced by the 

 expansion consequent upon the change of olivine, to serpentine. 



(f) Dunites. — Dunites are comparatively rare and show 

 no special features. They contain up to 80% of olivine, 

 with very subordinate amounts of enstatite and augite, and 

 always some picotite of early crystallisation. The struc- 

 ture in the slices examined is equigranular. 2 The alter- 

 ation of the olivine is mainly to serpentine, and in such 

 cases has been effected in the anamorphic zone. 



A tolerably fresh specimen was analysed with the follow- 

 ing result (A): beside it is placed an analysis of a dunite 

 from Dundas. 3 



1 Cf. Harker, " Petrology for Students/' p. 90; Teall, British Petro- 

 graphy, Plate viii, fig. 2; Twelvetrees and Pettard, f?oy. Soc. Tasmania, 

 1897, p. 28, 



- Cf. Iddings, "Igneous Rocks," Vol. i, p. 195, Vol. n, p. 321. 



3 W. N. Benson, Journ. Eoy. Soc. N.S.W., 1910, p. 582, 



