96 



approximates to an anamesite, intermediate between dolerite 

 and basalt. 



VII[. — Lherzolite. 



Rock 5. 

 Brown Lake. 



Occ^irrence. 



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

 which overlie the basaltic flow. They vary in size from a few 

 centimetres up to perhaps thirty centimetres in diameter. The 

 majority of the nodules have been enclosed in a thin layer 

 of tachylyte, and in some cases with vesictilar olivine basalt. 

 The specimens collected were selected from the most concen- 

 trated zone that could be found, viz., in a layer of nodules 

 in the ash-beds about 200 ft. above the water-level at the 

 southern boundary of Brown Lake. Many fragments and 

 nodules of bomb-like appearance occur, both as erratics and 

 in isolated layers in the ash-beds. They are certainly of 

 deep-seated origin, but the outer layer or coating is purely 

 volcanic. 



Macroscopic Characters. 



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

 grained, but coarse, containing allotriomorphic fragments of 

 rhombic pyroxene, which are dark-olive-green in colour, light- 

 green particles of diallage, and a light-coloured olivine, which 

 occurs in greater abundance than the former two minerals 

 and makes up the greater bulk of the rock. Occasionally 

 dark, glistening opaque grains are to be found, which were 

 proved to contain chromium when tested qualitatively, hence, 

 as in most other peridotites, are probably chromite or pico- 

 tite. The rock is extremely friable, and the constituent min- 

 erals can be separated very easily with the hand. 



The specific gravity of the rock is 3'33. 



Microscopic Characters. 



Owing to the extreme friability of the rock a microscopic 

 section was with difficulty prepared. The rock is hypidio- 

 morphic, even-grained, composed of olivine, enstatite, dial- 

 lage, and picotite. The pyroxene grains are only slightly 

 idiomorphic and moderately large, and in some cases are im- 

 bedded in the olivine, giving it a pseudo-porphyritic appear- 

 ance. 



The olivine occurs in great abundance, being the princi- 

 pal constituent of the rock. It has a high refractive index 

 and a strong double refraction. It is traversed by many irre- 

 gular cracks, and shows evidence of slight decomposition to 



