GEOLOGICAL SOCIETY. 



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them ; but it has got a confused dodecahedral cleavage, 

 not an hexahedral one, as Breithaupt observes, who is 

 the best authority, and so I think it should have been 

 mentioned, in the third order of the second class. 



I doubt if the harmotome is duly placed under the 

 right rectangular prism, which is easily reduced to the 

 oblique octahedron ; because on the twin crystals, the 

 faces of the pyramids concur exactly together, and they 

 would not thus concur if they were oblique octohe- 

 drons ; whatever it may be, the measurement of the an- 

 gle of bevelment is not 177° 5, but 110° 26 ; the con- 

 trary statement I suppose to be an erratum. 



After all, the most difficult part for the student is the 

 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. The 

 author observes, at page 70, " that if we arrive at the 

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

 independently of the cleavage, means for determining 

 the base, whether it be horizontal or oblique. " 



At first, I thought he alluded to the position of the 

 axis of double refraction in transparent crystals, in 

 which manner it has been determined that the euclase 

 is an oblique rectangular prism, or a right rhomboidal 

 one ; but I found no allusion concerning polarization in 

 his mineralogy ; and he is very right not to entangle 

 the poor student in this tortuous labyrinth, since it 

 would be the shortest way to send him at once to Bed- 

 lam. Afterwards I found the means suggested at pages 

 77 and 78; but I know not if they are adequate for people 



