382 MM. W. C. Broffffer and G. vom Bath on certain 



oo 



Considerable difficulties present themselves in the optical 

 investigation of the Kjorrestad enstatite, arising from the 

 perfect cleavages and the imperfect transparency. Professor 

 Des Cloiseaux had the kindness to undertake the research, and 

 determined the crystalline system from the optical character 

 to be rhombic. It was done at a time when we were still in 

 doubt concerning the form, and especially as we had not yet 

 reduced the forms of the Kjorrestad crystals to those of bron- 

 zite and enstatite. 



According to Professor Des Cloiseaux, the optical axes lie in 

 the brachypinakoid. The acute positive bisectrix is parallel 

 to the edge of the vertical prism. A plate cut normally to this 

 edge gave (in oil): — 



o / 



Right hyperbola to the normal 37 30 

 Left „ „ 41 



2H fl . r = 78 30 



Most of the crystals, as already mentioned, show a remark- 

 able deformity, which consists in a dislocation or turning 

 (round the makrodiagonal as axis) of all the faces forming 

 the summit. The brachydiagonal becomes, to all appearance, 

 klinoaxis. Whilst the crystals show, in consequence of this 

 deformity, an asymmetric form on both sides of the macro- 

 diagonal plane, the brachydiagonal plane becomes the face of 

 symmetry. It is difficult to believe them of a rhombic cha- 

 racter on looking at only a few of these crystals. But exami- 

 ning more of them, we get convinced, even without measuring, 

 that the degree of the dislocation is different in each crystal, 

 and that the whole phenomenon is to be reduced to pertur- 

 bations. 



But all the crystals are not deformed : some of them are of 

 pure rhomboidal character; and these allow a crystallographic 

 identification with the enstatite of Breitenbach (following von 

 Lang, Sitzungsber. Ak. Wien, vol. lix. 2. Abth. Aprilheft, 

 Jahrg. 1869), and with the hypersthene from the Bocher du 

 Capucin in Montdore and from the lake of Laach (Des Cloi- 

 seaux, Manuel ale Minfralogie, vol. ii. pp. xiv-xviii ; vom Bath, 

 Pogg. Ann. vol. exxxviii. p. 529, and vol. clii. p. 29). 



In order to be able to compare the crystals of the three 

 above-mentioned localities, we have drawn in the figures 1, la, 

 2, 2<2, 3, 3 a the crystals of Breitenbach (meteoric), Laach, and 

 Bocher du Capucin. 



We choose as the primitive form the pyramid measuring in 

 the 



