368 Mr. Brooke on the Identity of Biotine and Anorthite, 



vithin the United States is a great desideratum, toward the 

 accomplishment of which the attention of American geologists 

 cannot be too sedulously directed. 

 April, 1837. 



LXXIV. Ow the Identity of two Minerals from Vesuvius 

 named Biotine and Anorthiiey and on a new Variety of 

 Hemitrope Crystal of Quartz, By H, J. Brooke, Esq.^ 



[With Figures: Plate III,] .^.;v-<.i:(/ /^ ^ f^ 

 1 N a paper in the last Number of this Journal, p. 278, on the 

 -■■ regular crystalline composition of two different minerals, 

 allusion is made to the combination of Felspar and Cleaveland- 

 ite in crystals, which, however, by an error of the press, are 

 said to be from Bavaria instead of Baveno, 



The exact relations of the crystals of these two minerals 

 and of Anorthite have not, that I am aware of, been exactly 

 pointed out so as to explain the combination referred to, and 

 I am therefore induced to request the editors to allow a space 

 for the figures alluded to below. 



Fig. 1. (Plate III.) is an oblique rhombic prism, now 

 adopted as the primary form of Felspar. The small figure 

 marked P M T, the faces of which are parallel to the cleavage 

 planes, was regarded by Haiiy as the primary, and I'rom the 

 relation which subsists between these two figures, the se- 

 condary forms of felspar might be derived from either. 



Assuming for the present purpose the small figure, en- 

 larged and represented by fig. 2, as the primary, the follow- 

 ing analogy and differences of angles will be found to subsist 

 between Felspar, Anorthite and Cleavelandite. 



P on T. P on M. T on M. 



Anorthite 86° 110° 40' 117° 30' 



Felspar 90 112 ]20 35 



Cleavelandite ... 93 30' 115 119 30 



The crystals of Cleavelandite are formed over the planes 

 T and M of Felspar, with their axes parallel to the intervening 

 edge. 



Fig. 3 represents a regular crystal of Anorthite, and figg. 

 4 and 5 two crystals of the variety named Biotine. The cry- 

 stal of Anorthite is lengthened proportionally in the direction 

 of its axis, and fig. 4 of Biotine very disproportionally in the 

 direction of the oblique diagonal. It appears from M. Monti- 

 celli's figures that he has considered the planes T, 7i, P, e, as 

 • Communicated by the Author. 



