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TRANSACTIONS OF THE 



unacquainted with the first rudiments of geometry, for 

 semicrystallized bodies of the second class, which must 

 be of course broken or imperfect crystals : what 

 Mr. Shepard takes for horizontal planes, another ob- 

 server will take for vertical ones, and there are many 

 examples. I was astonished to read at page 78, at the 

 end " crystals belonging to the doubly oblique prism 

 are among the most difficult to be understood, and the 

 student, in examining them, will generally apply di- 

 rectly to the cleavage, the knowledge of which, though 

 it be but in one direction, will often be sufficient to 

 enable him to distinguish the primary planes f but 

 the cleavage in one direction, gives nothing but paral- 

 lel planes, which afford no angles." I grant there- 

 fore the student to know two directions, P and M, 

 inclined to each other 93°, but I cannot grant him to 

 know the minutes of broken or imperfect crystals. 

 How can he distinguish Valencianite from Mexico 

 (my chovelia,) from the Perikline, perhaps better 

 proskline, or the albite? the crystals of Valencianite 

 are distinct but so imbedded, that my pupil Busta- 

 mante thought that P, andT, was inclined 124° 30' in- 

 stead of 122° 30': he was only mistaken in two degrees ; 

 I assure Mr. Shepard that his student will make great- 

 er blunders than mine, although well acquainted with 

 geometry. 



As the author pays as little attention as Beaudant 

 to the diagonal cleavages, he misses a good character 

 to distinguish Disthen, Sillimanite, and Jeffersonite, 

 which have a distinct, short or brachy-diagonal cleav- 



