and on a nem Crystal of Quartz, 369 



those of a prism distinct from Anorthite, and has assumed the 

 Others to be terminal planes. But the correspondence of the 

 planes marked by similar letters in the figures may be ascer- 

 tained by means of the reflective goniometer, so as to remove 

 all doubt of the identity of the two minerals. Many of the cry- 

 stals named Biotine are flat, and have apparently square ter- 

 minal planes as shown in flg. 4, which is a remarkably de- 

 ceptive crystal, and without reference to the goniometer might 

 easily be supposed not to belong to Anorthite. 



A crystal of quartz in the writer's cabinet, resembling fig. 8, 

 and said to have come from Dauphine, presents a hemi- 

 trope form differing from any hitherto described. In carbo- ^ ^ 

 nate of lime the axis of revolution of twi^n crystals is parallel "^ 

 or perpendicular to the crystallographic axis, or perpendicular ^f^9 

 to a primary plane, or to a tangent plane on a primary edge ; 

 and in all other cases it has been found parallel or perpendi- 

 cular to an axis, or perpendicular to a primary edge or plane, 

 or to a single plane produced by some simple law of decre- 

 ment. Indeed so universal have these relative positions of 

 the axes of revolution been in all the instances hitherto de- 

 scribed, that they have been regarded by Haiiy and others as 

 fundamental laws of this kind of structure. Haiiy's expres- 

 sion (Cryst., vol. ii. p. 273) is, " Le plan qui est cense avoir 

 partage le crystal original en deux moities est toujours paral- 

 lele, soit a une des faces du noyau, soit a une face produite en 

 virtue d'une loi simple de decroissement sur les bords ou sur 

 les angles du meme noyau." 



Tlie axis of revolution of the crystal fig. 8 is perpendicular 

 to one of a pair of planes replacing an edge of the primary 

 rhomboid, resulting from a complicated, intermediary law, ex- 

 pressed, according to the notation of Haiiy, by (B 1 D^ Dj)> 

 and which would produce tangent planes on the edges of the 

 pyramid of the common crystals of quartz. Fig. 6. shows the 

 position of these planes «, a\ a'\ on the primary rhomboid ; 

 and fig. 7. shows their relation to the hexagonal pyramid. 

 These planes a do not occur on the hemitrope crystal, fig. 8, 

 but as the axis of revolution is perpendicular to the edge dcy 

 it must consequently be perpendicular to a tangent plane re- 

 placing that edge. 



Assuming 94° 15' as the angle of the rhomboid of quartz, 

 the inclination of the e(\ge dc of the pyramid on the axis will 

 be 42° 16', and consequently the angle d e d' will be 84° 32'. 



H. J. B. 



Third Series. Vol. 10. No. 62. il% 1837. 3B 



