CHAPTER VI. 



ON SERPENTINE. 



HISTORICAL SKETCH. 



IF we were to adhere strictly to our plan of arrangement we should 

 refer in this connection only to those serpentines which can be proved 

 to have been produced by the alteration of eruptive rocks belonging to 

 Group A. Having regard to the difficult nature of the subject and to the 

 imperfect character of our knowledge, it seems desirable, however, in this 

 brief historical sketch not to limit ourselves rigidly to the serpentines which 

 have been definitely proved to be of eruptive origin. 



Serpentine occurs as a mineral and as a rock. The mineral serpentine 

 3 (Mg Fe) 0. 2Si0 2 . 2 H.O., whenever it shows definite external form 

 appears to be a pseudomorph. In 1835, QUENSTEDT (1 ' described the so-called 

 serpentine crystals of Snarum, in Norway, and arrived at the conclusion that 

 they have been produced by the alteration of olivine. His view as to their 

 origin met with some opposition from SHEERER, HERMANN and others, but 

 it has been fully established by G. ROSE (2) and VOLGER, (S) as well as by a 

 host of more recent observers. 



These crystals frequently contain unaltered olivine. The relation 

 between the serpentine and the olivine has been well described by G. ROSE, 

 and his description will apply without any alteration to the appearances seen 

 in microscopic sections of altered olivine-bearing rocks. The original olivine 

 is surrounded by a border of serpentine, and this substance also occurs on 

 either side of the cracks which traversed the original crystals. As a result 

 we have kernels of olivine embedded in an irregular net-work of serpentine. 

 The following analyses by HELLAND illustrate the nature of the change. 



(1) T.A. Vol. XXXVI. p. 370. 



(2) P.A., Vol. LXXXIL, p. oil. (1851). 



(;5) Etitwiokeliings gescliichte der Talk-glimmer Familie, p. 283s 



