W. N. Benson — Origin of Serpentine. 697 



in detail the association with the serpentine of radio- 

 larian rocks, first noted by Bonney (1879). Mazzuoli 

 and Issel (1881), also Lotti (1883), following the sugges- 

 tion of Daubree (1879), held that the lava had been 

 erupted in a very hydrous condition. The associated 

 gabbros were not considered igneous, but altered sedi- 

 ments and volcanic muds. 



StapfT (1880) noted in the case of the serpentine of 

 the St. Gotthardt, that its boundaries sometimes follow 

 the stratification of the neighboring rock, but sometimes 

 go across them, but added that there is no proof of the 

 penetration of the serpentine-mass into the rocks encas- 

 ing it. "Although we would not consider the serpentine 

 to be an intrusive rock, we must remark that it could not 

 have had precisely the same sedimentary origin as that 

 supposed for the 'micaceous gneiss which encloses it. 

 We may regard it as originally a deposit of hydrated 

 silicate of magnesia formed by springs, and enclosed 

 between the sediments which gave rise to the mica- 

 schists." The hydrated magnesian silicate is supposed 

 to have been subsequently converted into anhydrous 

 olivine, etc., which by a later hydration has generated 

 serpentine. The apparently intrusive features are 

 explained as the result of earth-folding acting upon 

 structures with different powers of resistance. (Cited 

 from Hunt, 1883.) 



A fact often mentioned in this discussion was the fre- 

 quency of the association of serpentine and limestone 

 in such a manner as to suggest genetic relationship, that 

 e. g. the serpentine had been produced by the action of 

 silicifying solutions upon magnesian limestones, with the 

 production of olivine and peridotite, which subsequently 

 became hydrated. There is no doubt that such a process 

 has often occurred, notably in the case of some ophical- 

 cites, and the Eozoon rocks, but the serpentine produced is 

 not of the normal character; it is less dense, has a pale 

 honey-yellow color, is poor in iron and free from chromite 

 and nickel. These distinctions were recognized by Hunt 

 (1883), who, however, believed that all serpentines were 

 originally precipitates formed in pre-Cambrian times on 

 the floor of a primordial ocean, and that masses of appar- 

 ently intrusive serpentine, e. g. those among the Creta- 

 ceous and Eocene rocks of Florence, were to be considered 

 as inliers. The Italians recognized several ages for the 



