Rejoinder of Prof. Shepard to Prof Del Rio. 133 



tive form. A student who is even moderately acquainted with 

 the connexion of forms, would be prevented by the difference of 

 lustre on the pyramidal faces of most Quartz crystals, no less than 

 by the striae on the alternate faces of the prism, from referring them 

 to the order of the regular hexagonal prism : and as to the fact of 

 the pi'imitive form being among the actual crystals of this species, 

 it is abundantly mentioned as occurring at several places in Europe, 

 by authors of the highest authority, and I should be extremely hap- 

 py to show Professor Del Rio samples fi'om Chesterfield, Mass. 

 in my collection, (fig. 360, my Mineralogy, 2d part,) samples which, 

 though not the unaltered rhomboid, are so far removed from the six 

 sided prism, as to require an expert observer to detect in all instan- 

 ces even the rudiments of prismatic planes. 



Had Prof. Del Rio been as explicit in his first review of ray 

 treatise, as he with some want of candor claims to have been in his 

 notice of my reply, I should no doubt have extended my remarks 

 in commenting upon the discoveries of Mitscherlich, in a man- 

 ner more answerable to his expectations. The doctrine of dimor- 

 phism, I regard as too imperfectly established to justify any innova- 

 tions among species founded on natural-history principles. Chem- 

 ists may by making crystallizations in different menstrua and at va- 

 rious temperatures, obtain irreconcilable forms of what is supposed 

 to be the same substance ; they may fail also to detect any chemi- 

 cal difference between Flos-ferri and Calcareous Spar, and between 

 White and Common Iron Pyrites ; but still the interests of Mine- 

 ralogy will not permit the union of these substances, differing as they 

 do in crystalline form and other natural properties. The history of 

 chemical analysis during the last twenty years, forbids such a proce- 

 dure. The evidence of difference arising out of structure, specific 

 gravity, hardness and lustre, must still be preferred to that derived 

 from chemical analysis. 



While the announcement that Arfwedson has just found 37 p. 

 0. of sulphur in the European Manganblende, is a striking cor- 

 roboration of the suspicious value I would attach to chemical anal- 

 ysis, I am compelled still to disagree with Prof. Del Rio respect- 

 ing the identity of the Mexican variety with it as a species. The 

 discrepancy of form, if real — so great as that of a cube and a rhom- 

 boid — is enough to induce me to make a mineralogical distinction. 



The broken crystal with vertical planes, proposed as a dilemma 

 for my characteristic, may contain such faces as to render it certain 



