323 Mr. Shepard's reply to Prof. Del Rio. 



in other physical properties, constituting each a distinct species. So 

 that in neither case, can the shghtest confusion arise in making the 

 primary forms of Brooke, or indeed those of any crystallographer 

 whose writings I am acquainted with, the grounds of an analytical 

 system. 



But while the discoveries of Mitscherlich are thus shown to 

 offer no obstacle to a crystallographical system, 1 must leave it for 

 Professor Del Rio to reconcile them with the chemical distribution 

 which he has thought fit to adopt in his own recent Treatise on Min- 

 eralogy,* or indeed with any other chemical arrangement yet devi- 

 sed, or, (I may add) conceivable. 



Of the proposition to have every secondary form the ground of 

 an order, I have only to remark, that it is as extravagant, as is un- 

 natural the concluding member of the sentence containing it, and 

 which relates to the impossibility of the student's ever discovering 

 that Quartz belongs to the order of the rhomboid, of the truth of 

 which my early experience in referring the rhombic crystals of the 

 species in question, from the albite granite of Chesterfield, Mass. to 

 this order, may perhaps afford a suitable comment. But with respect 

 to the correct reference of uncleavable, regularly six-sided prisms, 

 terminated by pyramids, to which I suppose allusion is mainly had 

 in the remark, any person who has studied the operation of the 

 law of symmetry in producing new forms from the primaries, would 

 be able to know that these must have for their system of crystalliza- 

 tion, either the rhomboid, or the regular hexagonal prism. 



Of the correct reference of Harmotome to the order of the right 

 rectangular prism, I have by no means felt certain; but of the fact 

 that the pupil would with the utmost difficulty be led to place 

 it under either of the other orders I cannot doubt ; and indeed the 

 evidence in favor of its being correctly placed where it is, in pref- 

 erence to referring it to the octahedron with a rectangular base as 

 suggested by Professor Del Rio, (if I understand him correctly), 

 is strengthened from the fact that the cleavages are more obvious, 

 parallel with the lateral, than with the pyramidal, faces. The erra- 

 tum of angles imagined to exist in the Treatise, only existed in the 

 manner of reading, as the angle of 177° 5' is not set down to 

 Harmotome, but to Comptonite ! 



But to continue with the observations of Professor Del Rio, — 

 "after all," he says, "the most difficult part for the student is the 



* Elementos de orictoErnosia 6 del conocimiento de los fosiles. Filadelfia. 1832. 



