1885. J 



371 



[Genth and Vom Rath. 



" Fig. 2. The crystals reach 1'"" in size, 

 " they are highlj'' aggregated, sometimes so 

 " thickly and in such subparallel position 

 "that the faces seem to be brol^en. The 

 " best specimen of this kind gave the fol- 

 " lowing results : 



" o : o (brachydiagonal edge) = 126° 45' 

 " o : m = 14f3 24 



" Considering the minuteness of these crys- 

 " tals and the fact that the reflection is not 

 "sharply defined, this agreement can be 

 " regarded as sufficient. They are attached 

 " toa quartzilic gangue and of columbine 

 " red color. 



" Fig. 3 shows crystals of larger size, 

 " reaching 8™™. The faces are somewhat 

 "uneven, blackish-red, opaque; the plane 

 "a striated by oscillation with f. 



" Fig. 4. Wonderfully fine crystals with 

 " the dominant prism m. Their size reach- 

 "es 3""". Implanted on a quartzitic gan- 

 "gue Willi pyrolusite. 



"There is a distinct cleavage parallel to 4< 

 " face b, producing a reflection from tlie in- 

 "terior of the crystals in this direction. 



"Regarding the optical properties of des- 

 " cloizite I take pleasure in communicating 

 "the following lines from a letter of M. Dea 

 " Cloizeaux, dated March 18th, 1885 : 



" 'Les anciens petits cristaux de Cordoba, 

 "portant presque toujours la base, m'avaient 

 "fourni plusieurs lames minces bien paral- 

 "leles a cette face et qui paraissaient per- 

 " pendiculaires a la bissectrice negative, avec 

 ' ' plan des axes parallele a la petite diagonale. 

 "Mais les axes sout si ecartees autour de 

 " cette bissectrice et la dimension des lames 

 "si exigue qu'il nem'a jamais ete possible 

 " d'apercevoir lesanneaus dans I'huile. A travers les lames h', perpendic- 

 "ulaires aux premieres, mais d'une orientation necessairement moins cer. 

 "taine il est ^galement impossible de voir les anneaux autour la bissectrice 

 ^'positive. On pent done seuletnent dire que les bissectrices paraissent avoir 

 " I'orientation qu'elles auraient dans un prisme rhombique, mais il manque, 

 "pour avoir une certitude presque absolue la constatation de I'absence ou 

 " de I'existence d'une dispersion inclineo. Quant aux nouveaux cristaux 



