310 Royal Society. ./ ~ ! 



should be retained for the sodolithic species first discovered at 

 Penig by Breithaupt ; and the white or yiolet-tinted lamellar masses 

 abundant at Montebras will be included under it ; the hydrated 

 and entirely lithic species, comprising the laminar specimens and 

 the crystals from Maine, as well as some greenish masses from 

 Montebras, should be embraced under the name montebrasite. 



The amblygonite of Montebras has only been met with in 

 larninar masses with a faint tinge of violet. These masses exhibit 

 two cleavages presenting nearly the same degree of facility, 

 making with one another an angle of 105° 44'. Close observation 

 shows that the sharpness of the reflected linages is generally a 

 little greater on one of the cleavages than on the other; and 

 this induces one to suppose that they do not both belong to equivalent 

 crystallographic planes. The study of some of their optical pro- 

 perties, though presenting certain special difficulties, arising from 

 the small extent of the transparent portions and the presence of 

 numerous twin plates, even in the specimens that to all appearance 

 are the most homogeneous, has proved that the laminar masses of 

 montebrasite must be referred to the triclinic system. The optic 

 axes are situated in a plane which divides into two very unequal 

 parts the acute angle of 74° 16' of the two cleavages. This 

 direction is entirely different from that found for montebrasite of 

 Hebron and of Montebras, in which the plane of the axes lies in 

 the obtuse angle of 105° formed by the two principal cleavages. 



The appearance of the bars traversing the central ring of each 

 system indicates very distinctly a twisted dispersion, as well as a 

 small amount of inclined dispersion, which is characteristic of a 

 ciwstal belonging to the triclinic system. 



In November 1871 the author received a specimen from the 

 middle of a mass of amblygonite from Montebras resembling 

 the mineral from Hebron. It has three principal cleavages, _p, 

 m, t, which the author recognized in the mass from Hebron, 

 the angles between which are p m = 105°, mt=13o° to 136°, 

 p r=89° to 89° 15'. 



By means of artificial twins formed of two plates, each of which 

 had been worked perpendicular to the two cleavages p and m, and 

 which were united by their faces p, it appeared that the plane of 

 the optic axes is situated in the obtuse angle p m, and traverses 



the edge ^-, but that it is not quite normal to m, since it gives 



angles of about 82° with m and 23° with p. The character of the 

 coloured rings shows that in montebrasite of Montebras, as in that 

 from Hebron, there coexists with the horizontal a well-marked 

 inclined dispersion ; and these are peculiar to crystals of the 

 triclinic system. 



