195 



tion (until the system H,0— MgO— FeO— Fe,0, — SiO, shall be 

 investigated) that the magma, from which this rock originated, 

 crystallized under such a pressure that the gaseous components 

 (notably watervapoui) could not escape and consequently were taken 

 up into the rock substance from the very beginning of tiie crystal- 

 lization, thus occasioning a primary origin of serpentine. Putting 

 it chemically'): the crystallization begins in the serpentine-field and 

 terminates in a point serpentine-olivine (magnetite?), which is per- 

 haps located close to the connecting line olivine-serpentine (because 

 Fe,0^ takes up only Utile space in an olivine-serpentine structure). 

 In the case of eutecticum this point will be found on the same 

 side as Fe,0,, and in that of resorption in the common field of 

 serpentine-olivine. 



It may be suspected that in other peridotites, in which olivine 

 crystallized first, the said pressure was less, so that, indeed, the 

 gases could escape at the beginning of the crystallization, but were 

 taken up again afterwards at the final crystallization, so that in 

 similar cases serpentinization of olivine might be considered as an 

 apomagmatic (hydrolhermal) process. Expressing it chemically : the 

 crystallization then begins in the olivine field ; on increasing pressure 

 the stability field of the olivine is subsequently left for that of the 

 serpentine. The consequent segregation of magnetite is self-evident 

 after what has been said before. Magnesium is also set free for the 

 forming of periclase or picotite or magnesite. As the gases move 

 upwards it is obvious that serpentinization will occur chiefly in the 

 upper zones of peridotite-masses and on rents in the solidifying and 

 consequently shrinking peridotite-masses. 



Erosion being a downward process, first the marginal portions 

 are laid bare, so that in the field the serpentine will in many cases 

 be found prior to the olivine, which fact, 1 think has lent support 

 to the erroneous but current view that serpentine is a weathering 

 product. 



After the foregoing had been written (August 1922), the chemical 

 analysis came to hand (Dec. 29). 



Of a sample freed as much as possible from magnetite an analysis 

 was made at the Head Office of the Mining Department by Mr. A. 

 TER Braake and Mr. G. J. Wally. The loss of water has been 



^) To simplify matters it has been assumed that the serpentine and olivine 

 are very definite compounds, which is not the case, of course. For the thermal- 

 pressure-diagram of the five-substance-system a six or seven- dimensional space 

 would have to be used, which would not facilitate the conception. 



