W. N. Benson — Origin of Serpentine. 727 



along the crevice by circulating partly meteoric waters 

 acting at no great depth. (This conclusion is based on a 

 re-examination of material previously described. Ben- 

 son 1910.) The distinction between the colored pleo- 

 chroic minerals without secondary magnetite, and the 

 colorless minerals with secondary magnetite produced 

 from the same rock, is so clear that the conditions of 

 formation of the two can hardly be the same. We may 

 freely admit that iddingsite and bowlingite are the 

 products of true weathering, but there does not seem to 

 be sufficient evidence to refer the formation of antigorite 

 or chrysotile to the same process. For this reason the 

 writer doubts the propriety of considering iddingsite to 

 be a ferruginous form of antigorite, owing to the differ- 

 ent mode of occurrence. It would be interesting in this 

 connection to study the changes of olivine in lavas acted 

 upon by solfataric waters, though on general consider- 

 ations the presence of abundant oxygen and the absence 

 of any noteworthy pressure would probably result in the 

 action being an intensified process of weathering. 



VII. Summary and Conclusions. 



The various lines of inquiry we have endeavored to 

 follow support the general view in regard to the large 

 ultrabasic masses that the chrysotile or antigorite-ser- 

 pentine of which they are composed is an alteration 

 product of an originally intrusive peridotite, often more 

 or less pyroxenic, and that in some cases at least the 

 hydration was brought about by the agency of waters 

 emanating from the same magma that produced the 

 peridotite, though not generally until a considerable 

 amount of further differentiation has taken place. The 

 change was, however, completed by the end of the one 

 orogenic period of vulcanicity. We have yet to explain 

 satisfactorily the absence of hydration in certain cases. 7 



It is not so clear that a peridotite which has escaped 



7 As a suggestion to this end alternative to that previously advanced, pp. 

 707-8 we may note Van Hise's view (1904, p. 350) : "Alterations of ser- 

 pentine in the zone of anamorphism are not recorded. But the general 

 absence of serpentine in the schists and gneisses of sedimentary origin 

 profoundly metamorphosed in the zone of anamorphism is conclusive evi- 

 dence that the serpentine that was once in these rocks, and the associate 

 secondary minerals, have recombined to produce heavy minerals of the 

 classes from which the serpentine and those other secondary minerals were 

 originally produced." This recalls Stapff's hypothesis of the origin of 

 olivine from serpentine, but does not seem to the writer to be convincing, 

 since it assumes without proof the prior development of serpentine. 



