composition it possessed affinities with uieiouite or zoisite, uiid Dr. HUNT 

 considered that certain varieties might be regarded as compact zoisite. 



In 1883, CATHREIN (1) published an account of some elaborate chemical 

 and microscopical researches into the nature of certain saussurites. He found 

 that by the use of very thin sections and a magnifying power of 300 diameters 

 the substance, which appeared cloudy and indistinct when viewed in thick 

 sections and with low powers, could be resolved into distinct minerals. Thus, 

 in one variety, prismatic microlites and irregular grains of a colourless or 

 greenish strongly refracting mineral were seen to lie embedded in a clear 

 groundmass. The prismatic microlites showed a six-sided cross-section, and were 

 often terminated by two planes meeting in a roof-like manner. The prisms 

 showed transverse jointing and a cleavage parallel to their length. They gave 

 straight extinction and their outlines were strongly marked, thus indicating a 

 mineral with high refractive index. The faces of the prisms were in some 

 cases so well developed as to admit of the measurement of the principal angles. 

 The crystallographic forms represented were u (021), m (110), b (010) and 

 a (100). The angles were u : u = 69 ; u : b = 55 ; m : m = 64. 



Now, as these angles and all the other characters above mentioned agree 

 with zoisite, there can be no doubt that the prismatic microlites belong to this 

 mineral. The colourless groundmass in which the zoisite-prisms are 

 embedded occurs in the form of irregular grains which sometimes show the 

 twin structure of plagioclase. 



In some of the saussurites examined by CATHREIN the prismatic crystals 

 were yellowish green, and possessed a marked pleochroism (yellow for rays 

 vibrating parallel to the long axis, and green to colourless for rays vibrating at 

 right angles to this axis). In these characters and also in crystalline form 

 this mineral agreed with epidote to which it was referred by the author. He 

 considers, however, that no distinction can be drawn between the zoisite-bearing 

 and epidote-bearing saussurites. The plagioclase associated with the zoisite 

 and epidote was determined in one or two cases to be albite. Having established 

 the fact that the saussurites investigated b}* him were mixtures of zoisite or 

 epidote and felspar, CATHREIN proceeded to calculate the relative proportions of 

 the different constituents from the bulk analyses of certain saussurites, taking 

 TSCHERMAK'S formulaoas the basis of his calculation. The following case of 

 a saussuritc occurring as a constituent of gabbro in the valley of the 

 Wildshonau (Tyrol), will indicate" the general nature of his results. 



Orthoclase 8'0 



Albite ... ... 41-0 



Anorthite ... 1-7 



Epidote ... ... 7-6 



Zoisite 41-7 



100-0 



(P Ubor Saussurit. 7.K., Bund VII.. 1S8M, p. 2^1. 



