OF ARTS AND SCIENCES. 225 



OBLIQUE ZONE. 

 DES CLOIZEAUX. 



lOlAlTl 31° 53' 30" 31° 55' 



ITIATTT 11G° 13' 116° 18' 



TTTATOT 31° 53' 30" 31° 50' 



The planes of the main prism zone parallel to the ortho-diagonal, 

 and those of the cliuo-diagonal zone, were very badly striated, and 

 somewhat rounded, so that most of the measurements were made from 

 mere flashes, only three of the angles being sharply defined. The 

 remaining three zones were more sharply defined, and it will be 

 noticed that in these zones the measurements more nearly coincide 

 with those of Des Cloizeaux, and in every case they coincide within 

 the probable limit of error due to the irregularity of the planes. 



Quartz Twins. 



In the September number of the American Journal of Science, Prof. 

 TV. G. Brown of the University of Virginia describes a group of 

 twin crystals of amethystine quartz from Albemarle County, Virginia, 

 which appear to resemble in structure twins originally described by 

 G. Rose,* and which he interpreted as twinned parallel to the funda- 

 mental rhombohedron, although united by a plane perpendicular to 

 the same rhombohedral face. It appears, however, as originally shown 

 by Nauman,t that the structure of these groups might be interpreted 

 as twins of the ordinary type, parallel to a rhombohedral face, — f R, 

 and the group examined by Professor Brown would seem to confirm 

 this view, in so far as the corresponding rhombohedral faces of the 

 twins are not absolutely in the same plane; although the irregularities 

 he describes may obscure the small angular difference of twenty-six 

 minutes, which could alone furnish decisive evidence as between the 

 two theories under discussion. 



Professor Brown also refers to the view of Ecle,1: strengthened by 

 the observations of Rath and Frenzel,§ and that of E. S. Dana.|| that 



* Poggcndorf's Annalen, Ixxxiii. 401, 1851. 

 t Elemente tier Mincialogie, 6th ed., p. IDO, 1864. 

 X Zeitsclir. d. deutscli. geol. Gesellscliaft, xviii. 428, 18G6. 

 § Poggendoif's Annalen, xvii. 155, 1875. 



II American Jour, of Science, III., xii. 448, 187G, and Zeitsclir. fur Krystall., 

 i. 39, 1877. 



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