preserved in Chalcedony » 513 



so decided, that no mode of crystallization or inorganic arrangement 

 can be conceived capable of imitating it. 



While on this subject it may not be useless to notice the various 

 modifications under which chlorite when mixed with chalcedony 

 presents itself. It is now I believe perfectly well understood (as I 

 'have already assisted in showing, in some observations on the island 

 of Rum) that the colouring matter of heliotrope is chlorite diffused 

 uniformly through the mass. From some specimens in my pos- 

 session, I think it probable that plasma is indebted for its colour to 

 the same substance; the different nature of the siliceous base alone 

 constituting the difference of aspect. In these specimens (brought 

 from Egypt) distinct grains of green matter may be observed in- 

 dependent of the general green stain which pervades the stone ; and 

 this probability is rendered still stronger from the colour and ap- 

 pearance Vv'hich is occasionally assumed by the more transparent 

 flinty matter associated with the chalcedony, the latter acquiring 

 from the chlorite the aspect of heliotrope, while the more trans- 

 parent parts put on the semblance of plasma. But of a substance 

 so rare it is necessary to speak with hesitation. In quartz the chlo- 

 rite is well known often to assume a contorted appearance resem- 

 bling the intestines of animals, but in which the regular crystal- 

 lization and superposition of the plates can be easily traced. Similarly 

 contorted fibres occurring in chalcedony do not exhibit the crystal- 

 lized structure, but appear to consist of very minute or of invisible 

 particles, or even of a mere colour diffused in that particular form. 

 At times the coloured fibre contains grains of chlorite attached to 

 its sides or interspersed through its course, thus emulating the im- 

 bedded seeds or jointed appearance of real confervse, and I have 

 reason to think often mistaken for them. I am uncertain whether 

 I should rank under the head of chlorite certain contorted fibres 

 Vol. II. 3 T 



