EED COLOB OF CAMBRIAIS^ AOT) OAI^BONIFErtOUS. 63 



proportion of this material ranges from about one-lialf of one per cent to 

 as niiicli as five per cent hj weiglit of tlie mass, Asfurtlier oxidation of 

 tliese crystals of magnetic iron goes on but slowly, tlie deposits in wliic^i 

 they finally come to rest may contain a large sliare of the material which is 

 to l)e, by further change, dissolved and distributed througli the bed. Even 

 where the ferruginous matter has been accunndated between tlie fragments 

 of rock in the form of a thin vein or coating separating the hits, it may l)y 

 a further process >^f change lose the iron in a greater or less measure and 

 become a mere stain. 



