8f> 



Owing to the absence of any well marked cleavages the cracks which 

 are seen to traverse the olivine in thin sections are usually irregular. Traces 

 of the brachypinacoidal (010) and more rarely of the macropinacoidal (100) 

 cleavages may, however, sometimes be detected.' 1 ' When viewed under 

 the microscope the surface shows a roughness which is brought out in 

 consequence of the high refractive power of the mineral. A difficulty 

 sometimes arises in distinguishing between olivine and enstatite when both 

 minerals are present in the granular form and when the latter does not 

 possess the characteristic prismatic or pinacoidal cleavages. In such cases the 

 colours produced under crossed nicols may be useful in effecting a diagnosis 

 when a considerable number of sections of both minerals are available for 

 comparison. The double refraction of olivine is much stronger than that of 

 enstatite ; so that the polarisation tints of the former occupy, on the average, 

 a much higher position in NEWTON'S scale than those of the latter, when 

 1 lie sections compared are of uniform thickness. A micro-chemical test is also 

 available for distinguishing between the two minerals. By treating the 

 uncovered slide with hot hydrochloric acid the oliviue is decomposed with 

 the separation of gelatinous silica while the enstatite remains unaffected. 



Olivine often contains inclusions of chromite and magnetite and occurs 

 itself as inclusions in the other minerals, especially hornblende and augite, 

 (see Plates III., V., &c.). In some of the peridotitcs of the Western Isles 

 of Scotland it contains the curious stellate and dendritic inclusions 

 which are regarded by Professor JUDD as of secondary origin, and also 

 minute liquid and gas cavities. The alteration of olivine by surface 

 agencies water, carbonic acid and oxygen gives rise to serpentiuous and 

 other pseudomorphs. The alteration to serpentine is highly characteristic. 

 It begins at the surface and along the irregular cracks which traverse the mineral. 



Figure illustrating the partial conversion oj olivine into serpentine. 



(1) REXARD. Report on the Petrology of the Rocks of St. Paul. The Voyage of 

 JJ,M,S. Challenge}-, Narrative, Vol. II., Appendix B., p. 9. 



