PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 17 



The, reason for this great diversity in composition lies mainly in the 

 fact that the serpentine rarely if ever occurs as an original deposit, but 

 is always secondary, a product of alteration of either eruptive or sedi« 

 mentary rocks rich in such magnesian minerals as olivine and the non- 

 aluminous pyroxenes. As, however, these rocks rarely consist of pure 

 magnesian silicates, but carry in addition lime, alumina, and various 

 metallic oxides, these constituents separate out during the process of 

 change, and recrystallize in veins, streaks, and blotches ascalcite, dolo- 

 mite, magnetite, etc., thus producing the common variations in color. 



The theory long ably advocated by Dr. Hunt to the effect that the 

 serpentine occurring intercalated with beds of schistose rocks and 

 limestones resulted from metamorphism of silico-magnesian sediments 

 deposited by sea waters is now very generally abandoned, and it is 

 doubtful if the substance ever occurs as an original deposit even in the 

 eozoonal forms. 1 



The following analyses will serve to illustrate the change in compo- 

 sition which takes place in the conversion of (i) olivine and (n) py- 

 roxene iuto serpentine. 



(a) Olivine, Snarntn, Norway ; (b) serpentine derived from the same ; (c) pyroxene, 

 Montville, New Jersey ; (d) serpentine derived from the same, aud (in) the theo- 

 retical composition of serpentine. 



This change, it will be observed, is, in the case of the olivine, simply 

 a process of hydration — an assumption of some 13 per cent, of water. 

 In the pyroxene the process is more complex and consists of a loss in 

 silica, of all the lime which crystallizes out as calcite, and an assumption 

 of nearly 14 per cent, of water. 



In the series exhibited the derivation of serpentine from a pyroxene, 

 as indicated in analysis n above, is admirably shown by the suite of 

 specimens from Montville, New Jersey. In the large mass placed out- 

 side the case the gray core of pyroxene may be observed covered with 

 a thin crust of serpentine aud traversed by large and small veins of the 



1 The reader is referred to " British Petrography," by J. J. H. Teall (Dulan and 

 Company, Soho Square, London), p. 104, for a most excellent historical sketch of this 

 subject. Also to Becker's report on the quicksilver deposits of the Pacific Slope. 

 Monograph xm, U. S. Geological Survey, p. 117. 



