Kpistilbitejrom Hetdandite. 239 



minerals were separated by certain constant differences in their 

 refractive power, or in their double refraction, or in the incli- 

 nation of their resultant axes, we might have expected that 

 some mineralogists would not appreciate the value of such phy- 

 sical distinctions ; but when we have shown that the physical 

 structure of the two bodies has entirely different relations to the 

 single cleavage which so strikingly characterizes both, we an- 

 ticipate the ready acquiescence of all classes of mineralogists. 



From this result, we may now return to consider the influ- 

 ence of Mr Levy's process on the union of mineral species. It 

 is obvious that the secondary form of one species may be de- 

 rived by calculation from the primary form of another species 

 which never exhibits the same modifications, and yet the two 

 species may be perfectly distinct ; and, consequently, the as- 

 similation of a real to a calculated secondary form can never 

 be held as a proof of identity of species. To this it may be 

 replied, that, in the case of Epistilbite and Heulandite, the as- 

 similation is not perfect, and that the principle may still be 

 sound, though its failure may, in the present case, be admit- 

 ted. This argument is plausible ; and we should certainly at- 

 tach some value to the principle, if all the other characters of 

 the two species put in comparison concurred in establishing 

 their identity ; but r the difference in the general appearance 

 of Epistilbite from the ordinary crystals of Heulandite," ad- 

 mitted by Mr Levy, — the existence of soda in the former, and 

 its superior hardness and specific gravity, — are points of dissi- 

 milarity which cannot be overlooked in estimating the present 

 application of the principle. 



Before concluding this notice, I cannot avoid directing the 

 attention of mineralogists to the singular perplexity into which 

 the crystallographer is thrown, by the discovery of Haytorite, 

 a substance found in Devonshire, possessing a distinct crystal- 

 line form, and exhibiting bright planes capable of having their 

 inclination measured by the reflecting goniometer. These forms 

 are considered by Mr W. Phillips and Mr Levy, two of our 

 ablest mineralogists, to be pseudomofyrfious, and the latter re- 

 gards it as probable that they have had these forms impressed 

 upon them in moulds, made either by crystals of Humboldtite, 



