63 



LHERZOLITE AND OLIVINE FROM MOUNT GAMBIER. 



By Evan R. Stanley. 



| Read August 2, 1910.] 



Plate XIX. 



Introduction. 



In a previous paper before this Society (1) mention was 

 made of lherzolite nodules occurring at Mount Gambier, but 

 having only a limited supply of the mat-erial in my possession 

 at the time I was unable to present a chemical analysis of the 

 rock. Since then more material has come to hand from that 

 locality, thus enabling me to make further petrographical 

 notes and analyses on not only the lherzolite, but the olivine 

 contained in it. 



Lherzolite. 



Occurrence. 



The rock occurs in the form of nodules in the ash-beds 

 overlying the basaltic now at Mount Gambier. They vary in 

 size from a few centimetres up to thirty centimetres or more 

 in diameter. In the majority of cases the nodules are coated 

 with a layer of vesicular-olivine-basalt or tachylyte. The 

 nodules examined were selected from the ash-beds in Brown 

 Lake, about 200 ft. above the water-level, where they occur 

 in great numbers. 



Macroscopic Characters. 



In hand specimens the rock is olive-green in colour, even- 

 grained, but coarse, containing allotriomorphic fragments of 

 light-olive-green olivine, light-brownish-green rhombic pyrox- 

 ene, dark-green particles of diallage, and black glistening 

 grains of chromite. 



The specific gravity of the rock is 3 '33 at 16° C. 



Microscopic Characters. 



Owing to the extreme friability of the rock a difficulty 

 was encountered in the preparation of microscopic sections, 

 but by cutting a fairly thick section and boiling it in Canada 

 Balsam, sections of suitable thickness were obtained. The 

 rock is hypidiomorphic, even-grained, containing olivine, 



(i) Trans. Roy. Soc, vol. xxxiii., 1909, p. 82. 



