Mr. Shepard^s reply to Prof. Del Rio. 319 



fess I am at a loss to know what is the signification of iota calo 

 different, when applied under such circumstances. Does this very 

 commonly occurring diversity of mechanical structure in the mem- 

 bers of almost every species constitute them such ? 



He proceeds to inquire, whether the same end would not be bet- 

 ter obtained, and triplicate trouble spared the student by adding to 

 the third table, that Galena is also found in cubes, and has a triple 

 perpendicular cleavage. I think not ; for the reason, that of mine- 

 rals crystallized, or massive in cleavable individuals, a specimen may 

 be determined by the two first classes with the slightest trouble in 

 comparison with what they could be, if they were to be sought for 

 blended up with the entire mass of the species, where the order of 

 succession is first hardness and then specific gravity : not to men- 

 tion that it would materially increase the labor of determining a 

 massive, uncleavable specimen, if the class appropriated to these 

 was burdened by all the characteristic details of crystallized minerals. 



" But granting" continues Prof. Del Rio " the partition into 

 three classes, as it is, Fluor is found most frequently in cubes, and 

 yet the student shall not be able to find it in the first order of the 

 first class, where it should be ; it is to be found in the third order, 

 the octrahedron." Where, I scarcely need add to the most care- 

 less reader of the Treatise, it could alone be, and where the most 

 inattentive student would not fail of referring it : for the order de- 

 pends upon the primary, which is not always the actually ocairing 

 form. And can it be imagined that the student would be misled in 

 settling the primary form of Fluor, when a cubic crystal of this 

 species cannot be turned in the hand without the discovery of its 

 octahedral cleavages, not to mention that few crystals can be hand- 

 led without jarring from their angles the pyramidal fragments which 

 conceal the faces of the primary octahedron. 



The suggestion of adding the trapezohedron as a primary form, 

 and of arranging under it the Leucite, Analcime and Garnet, does 

 not appear to offer any advantages : besides the objection which 

 the procedure incurs, that neither these nor any other minerals as- 

 suming this form, ever present trapezohedral cleavages, while all 

 which do, either give cubic, octahedral, or dodecahedral ones; and 

 the form in question flows from the cube, the regular octahedron 

 and rhombic dodecahedron by a very simple modification, in coin- 

 cidence with the law of symmetry. 



