Prof. Maskelyne and Dr. Lang's Miner alogical Notes* 43 



3. On the Crystalline Form of Lanthanite. By Viktor von Lang. 



It has been hitherto supposed that lanthanite crystallizes in 

 the pyramidal system ; but M. Descloizeaux found recently, in 

 the course of his important researches on the optical properties 

 of minerals, that lanthanite possesses two optic axes, the mean 

 line of which is perpendicular to the perfect cleavage plane. 

 The crystals of lanthanite must therefore be considered to 

 belong to the prismatic system. M. Descloizeaux, when in 

 England this year, had the kindness to acquaint us with his 

 discovery; and I tried in consequence to determine also the 

 crystallographic elements of lanthanite, the mineralogical col- 

 lection of the British Museum possessing small but tolerably 

 good crystals of this mineral from Bethlehem in Pennsylvania. 



The crystals were found to be combinations of a prism m (1 1 0), 

 a pyramid o (1 1 1), and the two planes Z>(010) and c (0 01). 

 As the plane c, which is at the same time a perfect cleavage 

 plane, predominates very much, the crystals have the form of 

 thin plates, the sides of which are bevelled by the small faces m 

 and o t as is represented in figure 5 of Plate II. 



Prom the measurements I made, the parameters of lanthanite 

 were found to be 



a:b:c= 1 : 0-9528 : 0-9468. 

 Prom these values the following angles are calculated : — 



Calculated. Observed. 



110.010=.°. 43 37 



110. 110 = 87 14 87 15 



1 1 . 1 1 1 = 37 24 37 26 

 1 1 1 . 1 1 T = . . 74 48 

 111.010 = 54 53 



1 1 1 . 1 1 1 = 70 14 

 1 1 1 . T 1 1 = 66 28 



4. On new forms of Mesotype. By Viktor von Lang. 



The minerals mesotype, mesolite, and scolezite all crystallize 

 in prisms with angles varying little from 90 degrees, and hence 

 present much similarity in their external appearance. Provided 

 the crystals be not too small, the separation of the three species 

 may easily be effected by the aid of the polarizing apparatus. 

 Scolezite is recognized by the small angle which the plane of 

 polarization makes with the direction of the prism. This angle 



