1825.] 



Mr. Levy on a neto Mineral. 



141 



measured by the reflecting goniometer, with the exception of the 

 plane A', which is strongly striated longitudinally. Some of 

 the crystals are opaque, and of a pale red colour ; others are 

 translucent and transparent, and of a deep orange red colour, 

 somewhat like the cinnamon stone. Fig. 4 represents a beau- 



Fig. 4. 



Fis. 5. 





h 



4. 



It; 



tiful crystal of this colour placed on a group of rock crystal in the 

 collection of Mr, James Sowerby. 



Upon a group of rock crystals from Dauphiny, in the collec- 

 tion of Mr. Turner, I observed with lamellar crichtonite some 

 flat very brilliant brown translucent crystals, the form of which 

 is represented by fig. 5, and which belong to the same species 

 as those above described; they present, however, new modifica- 



ti«ns which are the planes designated by y), e*, and e ; but all 

 the other planes m, h\ g\ and ei, measure exactly the same 

 angles as those mai'ked with the same letters in the crystals 

 from ISnowdon. 



I have taken for the lateral faces of the primitive form the 

 planes marked m, which are inclined to one another at an angle 

 equal to 100'^, and by assuming also that the planes marked e», 

 the incidence of which upon m is 

 equal to 134°, is the result of a de- 

 crement by three rows in breadtli on 

 the lateral angles e of the primitive, I 

 have found that one side of the base 

 was to the height nearly in the ratio 

 of 30 to 1 1 . A right rhombic prism, 

 fig. 1, of 100°, and of such dimen- 

 sions, may therefore, be considered as the primitive form of this 

 substance. The other planes are marked with the signs corre- 

 sponding to the decrements of which they are supposed to be 

 derived, and the incidences calculated from these laws agree 

 within very narrow limits with the observation. The faces 



Fig. 1. 



