W. N. Benson — Origin of Serpentine. 725 



few views may here be noted for the bearing they may 

 have on the origin of serpentine. Kalkowsky (1906) 

 noted the occurrence of nephrite in Liguria, a rock rather 

 than a mineral, associated with talc, serpentine and cal- 

 cite in the neighborhood of faults and dislocations. He 

 considered it to be formed under deep-seated condi- 

 tions by pressure metamorphism of the talc-serpentine- 

 carbonate rocks. He described specimens of nephrite, 

 which he considers to be pseudomorphous after chryso- 

 tile and after talc. Steinmann investigated the same 

 occurrences as were described by Kalkowsky (1908) and 

 his view is cited above, and concurs with the earlier 

 hypotheses which referred the formatiomto a process of 

 uralitization. Finlayson considered that both these pro- 

 cesses may have given rise to nephrite in New Zealand, 

 and adds another method, namely a direct transforma- 

 tion of olivine into nephrite. Bonney, however, saw 

 difficulties in this last and especially in the derivation of 

 nephrite from talcose rocks, believing that the reverse is 

 more probably the case. The derivation by uralitization 

 appears to be unchallenged (Finlayson 1909 and dis- 

 cussion), and this is held to be a deep-seated process 

 associated with dynamic action. 6 



VI. The Occurrence of Serpentine in Volcanic Rocks. 



Obviously, any consideration of the origin of the min- 

 eral serpentine should include a consideration of the 

 conditions of its occurrence in volcanic rocks. This 

 would require a lengthy personal investigation into the 

 microscopical and field-characters of the basic lavas, 

 which as yet the writer has been unable to undertake. 

 In the literature there are very frequent statements that 

 the olivines of basalts have been serpentinized, and that 

 owing to their ferriferous character, the serpentine is 

 pleochroic. Sometimes the occurrence of a platy pleo- 

 chroic "serpentine" is noted, and referred to iddingsite, 

 considered to be a ferriferous variety of antigorite, and 

 "[Thiemann (1909) gives an elaborate account of the 

 formation by weathering of iddingsite as a ferruginous 

 form of antigorite in the picrites of Saxony, drawing 

 comparisons between this and the features described by 

 "Weinschenk. Nevertheless, a close inspection of all 

 examples of altered olivine encountered by the writer in 



6 But see Duparc et Hornung. 1904. 



Am. Jour. Sci.— Fourth Series, Vol. XL VI, No. 276.— December, 19 18. 

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