726 W. N. Benson — Origin of Serpentine. 



volcanic and other rocks, causes him to view with some 

 caution the frequent statements of the presence of ser- 

 pentine as a product of weathering. Under the term ser- 

 pentine in its restricted significance, we are here consid- 

 ering only those forms of chrysotile and antigorite as 

 are considered e. g. by Bonney (1905). Such minerals 

 when examined microscopically are nearly colorless, and 

 have a double refraction not greater than -013. Many, 

 perhaps the majority of writers, however, include also 

 under the term serpentine various more cr less deeply 

 colored and pleochroic minerals, with fibrous or platy 

 habit and a double refraction nearly twice as great. 

 Thus Teall (1888, p. 189) refers to "a rich deep green 

 serpentine. ' ' A study of Lacroix's collection in Paris, 

 illustrating his Mineralogie de la France et ses Colonies, 

 showed that this colored fibrous form is the mineral he 

 classes as bowlingite, and considers to be merely the 

 fibrous form of the platy pleochroic mineral iddingsite, 

 and also places it with the serpentine-minerals. Wein- 

 schenk (1907) concurs and refers to it merely as a ferru- 

 ginous form of serpentine. So also does Harker (1904), 

 who carefully distinguishes between the effects of mag- 

 matic water upon a basalt (the production of zeolites, 

 etc.), and that of true weathering, the " serpentiniza- 

 tion" of the olivine. The serpentinous material pro- 

 duced is described as being of a pale green color, and 

 associated with a micaceous substance which forms the 

 bulk of the pseudomorphs and is comparable with idding- 

 site. These pseudomorphs may re-absorb part of the 

 secondary magnetite, and become deep green in color and 

 strongly pleochroic (op. cit. p. 35). In gabbros from the 

 same region (Skye) he has noted that complete destruc- 

 tion of the olivine gives rise to pseudomorphs of green 

 and yellow-brown serpentine, the secondary magnetite 

 being here also absorbed, while pilitic pseudomorphs and 

 iddingsite may occur. The writer has observed a block 

 of dunite in a volcanic breccia, partly changed into nor- 

 mal chrysotile- serpentine and secondary magnetite, trav- 

 ersed by a band an inch in width composed entirely of 

 bowlingite, with no secondary magnetite, but preserving 

 the outlines of the olivine crystals. In this instance it 

 seems likely that the fragment of dunite was a homo- 

 geneous or cognate xenolith in a basaltic breccia, that it 

 was partially serpentinized at depth, and later torn from 

 its position, included in the breccia, fractured, and altered 



