SERPENTINIZATION WITHOUT MINERALIZATION. 43 



CHAPTER VIII, 



SERPENTINIZATION EFFECTED IN DEPOSITS WITHOUT THE 

 INTERVENTION OF MINERALIZATION. 



As certain formations, such as coal, anthracite, dolomite *, &c., 

 are methylosed products, and nevertheless possess no evidence 

 of their having been otherwise than in the ordinary amorphous 

 rock- condition, it may be fairly expected that there are forma- 

 tions, closely related to the ophites, which have undergone 

 chemical changes without having been previously mineralized. 

 Many argillytes are known to contain silicate of magnesia, which 

 makes them more or less steatitic, talcose, chloritic, or serpen- 

 tinous, against which objections might be raised as to their 

 being allocated amongst typical ophites. 



Mr. J. Arthur Phillips has made known several cases of 

 Cornish " killas," which, through a decrease of alumina and 

 an addition of magnesia, departs from its normal condition, 

 and passes into a substance approaching to ophite f. In these 

 cases the rock approximately retains its original amorphous 

 condition, having, in most cases, little more than the ordinary 

 texture of slate or argillyte. Some of the agalmatolytes have 

 all the appearance of being similarly magnesiated formations. 



There seems to be no reason, then, why the serpentization 

 of killas, or any other argillaceous rock, may not become 

 so far advanced as to convert it into an ophite. We are, 

 accordingly, led to conclude that certain rocks of the kind, 



* Beds of argillaceous limestone are occasionally converted, as in Derby- 

 shire and other places, into rottenstone (essentially, aluminous) through their 

 calcareous portion having been removed by water containing carbonic acid. 

 And much of the alabaster of Northern Italy, the Tyrol, and the Thuringer- 

 wald may be methylosed limestone j but in this case sulphuric acid and water 

 have replaced carbonic acid. 



t Philosophical Magazine, 4th ser, vol. xli. pp. 87-10C (Feb. 1871). 



