134 



G. D. OSBORNE. 



(e) Lherzolites. — The llierzolites contain less olivine 

 than the harzburgites, and diopside dominates over ensta- 

 tite. While many features are to be found in this group 

 similar to those investigated in the last group, still one of 

 the most singular changes of olivine occurs in a lherzolite. 



While bowlingite and serpentine occur with no striking 

 features, a number of grains of olivine show a composite 

 pseudomorph, so to speak. This consists of bowlingite, 

 with bright interference colours, forming fibrous patches 

 or segments partitioning off the olivine grains, serpentine 

 which is sometimes developed at the periphery of the grains 

 or in cracks, and opal with characteristic low D.R.; the 

 opal is always associated with a puzzling brown decompo- 

 sition product of irregular shape. This material is opaque 

 with dark brown to black colour, and in places grades off 

 insensibly into the bowlingite. It preserves the original 

 cracks of the olivine, and in reflected light shows no 

 metallic lustre, and only the merest indications of the 

 common hydrated iron oxides. That it is an altered iron 

 oxide which originally separated anhydrous out in large 

 measure after the manner of alteration of olivines described 

 by Jndd and Zirkel, is the only explanation forthcoming at 

 present. This view is strengthened by the existence of 

 opal which is often associated with residual iron oxides in 

 olivine. A figure of this manner of alteration is given in 

 Plate VII, fig. 5. 



In contrast to the olivine enstatite is almost unaltered. 

 The (110) cleavage is very well marked and schiller plates 

 are absent, although the rest of the rock expresses the 

 various vicissitudes through which it has passed, comprising 

 the subjection to pressure and conditions in plutonic regions 

 and also circumstances favouring the infiltration of solu- 

 tions in the zone of weathering. Thin bands of fibrous 

 carbonate cross the crystals and a little talc 1 is developed 



1 Cf. Kosenbusch, 'Mikroskopische Phjsiographie/ Band I, II, p. 180. 



