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



second class : he must ascertain by the cleavage, if a rhombic prism 

 is right or oblique, since if he mistakes it he will never find the name 

 of the specimen." Instead of never finding the name, 1 beg leave 

 to substitute, that he would not find it always in the most direct man- 

 ner ; for if the mineral sought should happen to belong to the latter, 

 he would first be led to examine unsuccessfully, the ten species of 

 which the cleavage order of the right rhombic prism consists ; (the 

 order of the right rhombic prism preceding that of the oblique,) 

 and if on the other hand it should happen to possess a right rhom- 

 bic prism as its form of cleavage, it would be, so far as the brevity 

 of the analysis is concerned, immaterial, whether he had correctly 

 settled the direction of the axis, or not. The remark quoted in 

 this connexion from page 70 of the Treatise, "that if we arrive at 

 the knowledge of the lateral faces of a prism, we possess independ- 

 ently of the cleavage, means for determining the base, whether it be 

 horizontal or oblique" is undoubtedly true, as must be sufficiently ob- 

 vious from what has gone before relative to the symmetrical changes 

 of right and oblique prisms, but has no relation to the subject here, 

 inasmuch as it is intended to apply to simple minerals or regular 

 crystals. And the same inapplicability applies to the light which 

 Professor Del Rio supposed he had obtained from pages 77 and 78, 

 since they treat exclusively of perfect crystals, as the Treatise plainly 

 shows. 



He is astonished at my remark that the discovery of a single 

 cleavage would assist in enabling the pupil to distinguish the primary 

 planes in the complex forms belonging to the doubly oblique prism ; 

 and adds by way of placing me in the predicament of self nullifi- 

 cation, " that cleavage in one direction gives nothing but parallel 

 planes," (quoted from another part of the Treatise) which is un- 

 doubtedly true in the general process of obtaining cleavage crystals, 

 but not at all in the present case, where we have beside the parallel 

 planes, natural faces either perpendicular or oblique to them, from 

 all which it is clear, that a regular solid must result. 



He says that he " cannot grant him (the student) to know the mi- 

 nutes of broken or imperfect crystals. How can he distinguish Va- 

 lencianite from INlexico (my chovelia) froin the Perikline, perhaps 

 better Proskline or the Albite ? the crystals of Valencianite are dis- 

 tinct, but so imbedded that my pupil Bustamente thought that P 

 and T was inclined 1'24° 30' instead of 12-2^ 30' : he was only mis- 

 taken in two degrees; I assure Mr. Shepakd that his student will 



