On the Genesis of Some Scottish Minerals. 191 



The various stages in the formation of Scottish agates will 

 very well serve as a general illustration of what ensues. 

 These are found in vapour cavities, which vary much in both 

 form and size, and may be faulted or otherwise deformed 

 before they begin to be filled. They are usually closed in, 

 on all sides, by solid rock. These cavities are filled entirely 

 by solutions carried in by osmosis. The first solutions so 

 transported from the solid rock to the cavity below are 

 usually those of the ferromagnesian constituents, which are, 

 in the case under consideration, usually Augite. These solu- 

 tions gradually coat the walls of the cavity, probably as a 

 succession of thin films, which are deposited from very weak 

 solutions. They form what may be termed the " priming," 

 upon which subsequent coats of other substances are after- 

 wards laid. The " priming " varies in composition with the 

 nature of the rock from which it has been derived ; but its 

 nature, in general terms, may be said to be more or less allied 

 to Serpentine. Its colour is usually green, nearly that of 

 moss, or, better still, of an ivy leaf. Various names have 

 been given to this green "priming," but Dr Heddle (who 

 devoted much attention to these matters, and to whom the 

 present writer is indebted for much of his knowledge of this 

 subject) finally concluded, after long consideration, that the 

 mineral in question was Celadonite.^ Commonly the deposi- 

 tion of Celadonite at any given level below the surface 

 terminated at an early stage — and, indeed, in some cases the 

 deposit is so thin that it is practically absent altogether. 

 But, in other cases, its formation may have continued long 

 past the stage where it formed simply a " priming " to the 

 interior of the cavity. In some few cases the deposition of 

 this or an allied saponaceous mineral evidently went on until 

 the cavity was completely filled. It was occasionally de- 

 posited quite alone; while in many other instances it formed 

 a mechanical mixture, combined in various proportions with 

 chalcedonic silica. True Moss Agates were thus formed, and 

 where the mixture had been more intimate, Prase, Heliotrope, 

 or Bloodstone was the result. 



^ Dr Heddle preferred to spell this "Celedonite," in which form it appears 

 in the "Mineralogy of Scotland." 



VOL. XIV. N 



