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 strie 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 primitive 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 Dex Rro samples from 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 i instan- 
ces even the rudiments of prismatic planes. 
Had Prof. Det Rio been as explicit in his first review of my 
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 MirscHER.icu, 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 meastrua 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 otber 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 Arrwepson has just found 37 p. 
ce. 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. Dex Rro 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 
