Crystalline Forms of Hay tor tie. 308 



tinctly seen, and in which there is no dispersion of the passing 

 light ; but as they are particles possessing double refraction, 

 and as the double refraction varies at different parts of the 

 crystal, a ray of light emerging from one part of a particle 

 will not enter the corresponding part of the adjacent particle ; 

 and therefore there must be that dispersion and reflection of 

 the light which takes place in chalcedony. 



Upon submitting the Haytorite to the same scrutiny, I could 

 not discover the slightest difference between its optical struc- 

 ture and that of chalcedony. It consists of particles posses- 

 sing double refraction, but having their axes lying in all possi- 

 ble directions. Hence arises the dull fracture observed by Mr 

 Phillips, and also the want of regular cleavage, which cannot 

 possibly take place in a mineral so constituted. The cleavage, 

 therefore, which you mention as having been observed by Mr 

 Cole, cannot be a regular one, but must be one of those acci- 

 dental planes of separation, which are often observed in granu- 

 lar rocks, where the mass has no crystalline form. 



This opinion of the perfect identity of Haytorite and chal- 

 cedony, in so far as their internal structure is concerned, is 

 confirmed by the similarity of their fracture and specific gra- 

 vity, as stated by Mr Phillips, but this result only adds to the 

 difficulty of explaining the origin of the crystalline forms of 

 Haytorite. For as chalcedony has been found in so many 

 localities, and in such a great diversity of forms, without ever 

 exhibiting the slightest trace of crystalline faces, we are natu- 

 rally predisposed in favour of the opinion, that the crystalline 

 forms of Haytorite are pseudomorphous, or derived from some 

 other mineral. 



We shall therefore proceed to discuss the question, and a 

 most important one in mineralogy, whether the forms of Hay- 

 torite can be derived from any other mineral. It will be seen 

 from the preceding description of this mineral, that the crys- 

 tals of Haytorite are of the most perfect kind, and that the 

 faces are derivable, by the laws of crystallization, from an 

 oblique rhombic prism, as their primitive form. Hence it is 

 clear, that the faces could not have been imprinted upon fluid 

 chalcedony, by the faces of a number of other crystals sur- 

 rounding it. If such a thing were within the limits of possi- 



