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



315 



Characteristic of Briethappt. 



Adamantine lustre. Color red, 

 sometimes passing to brown, 

 rarely to yellow. Streak pale 

 brown to nearly isabella-yellow. 

 Primary form, Brachiax, tetra- 

 gonal Pyramedaedron, P=/o 

 i = 135° 22' 42"; 64° 56' 

 42''; 2P'=122° 24' 6"; 83° 

 58' 28". (64° 56' also 64° 55' 

 Br.) Cleavage, parallel with 

 the primary prism, perfect; to 

 its diagonals distinct ; with the 

 faces of the primary pyramids 



My Cliaractcristic. 



(order 7, class I.) No. 10=4-2 

 . . . 4-4, while both the others dif- 

 fer from this by an amount greater 

 than 0-5. It is therefore Rutile. 



If a cleavable variety, the hard- 

 ness alone decides it to be species 

 No. 8. (ord. 6. class II.) or Ru- 

 tile. 



If uncleavable, its Sp. Gr. re- 

 quires it to be compared with Nos. 

 225, 236, 237, 248, 252, 257; 

 259, 261 , 264, 265, 266, and 268, 

 (Class III.) in Sp. Gr. while its 



in traces. Fracture, uneven to lustre, color and streak decide it to 



be 264, or Rutile. 



conchoidal. H. 8-0 .. . 9-0 

 G. 4-249 MoHs. 4-250 to 

 4-291 Br. which detennines 

 the mineral sought, whether 

 crystallized, massive in large in- 

 dividuals, or massive in small 

 individuals to belong to the spe 

 cies Rutile Hard-Ore of Br. 

 or Rutile (trivial name,) of 

 Enirlish mineralo<rists. 



It is obvious from the foregoing trial of these Characteristics, that, 

 while both are sufficiently certain in effecting the purpose intended, 

 the advantage in brevity is on my side. A single glance at the 

 two columns shows, that the amount of reading and observation 

 required in the two cases is quite different, even leaving it out of 

 the account that my column has embraced the determination of the 

 three varieties, crystallized, cleavable and uncleavable, while what I 

 have given from Breithaupt would be necessary for the diagnosis 

 of either one of them. Other trials would exhibit the subject in a 

 similar light. 



It is objected, moreover, to my Characteristic that the name of 

 the mineral being determined, the student has still to learn another 

 classification in order to that disposition of the species adapted to a 

 comprehensive survey of the whole contents of the mineral king- 



