VAKIETIKS OF SERPENTINE. 109 



talline substances, but that colors are usually apparent in laminated and 

 fibrous varieties. According to Professor RosenbusLdi' the distribution of 

 color in jjolarized light varies with the structure, here in patches, there 

 sinuous or in parallel streaks. Especially where the structure is fibrous 

 and chrysotile-like the change of colors, though not strong, is uiuiiistakable. 

 Where the structure permits of optical examination the substance is found 

 to polarize light more or less strongly and to be biaxial, with a highly varia- 

 ble angle between the optical axes. Messrs. Fouque and Michel-Levy' pro- 

 nounce serpentine a colloid mineral without any proper action on polarized 

 light, although eminently susceptible of presenting the optical phenomena 

 due to pressure. Thus, in very thin sections the colors of polarization 

 affect very pale, bluish tints and the greater part of the substance does not 

 react between crossed nicols; but in thick slabs, on the contrary, the colors 

 are often vivid and brilliant. On the other liand, foliaeeous varieties of the 

 mineral have been described in a number of very important investigations 

 of the serpentinoid rocks which agree with Professor Rosenbusch's descrip- 

 tion. Rocks of this class were investigated by I\Ir. von Drasche and later 

 \)j Messrs. "Weigand,^ Becke,'* and Hussak,'' all of whom found them 

 mainly composed of foliaeeous, distinctly polarizing, serpentine varieties, 

 such as bastite, picrosmine, and metaxite. Mr. Hussak has described this 

 material minutely and referred it to antigorite. According to this authority 

 it has considerable pleochroism, is biaxial, and shows blue-gray tints be- 

 tween crossed nicols. The analysis is that of a somewhat ferruginous ser- 

 pentine. Mr, F. Eichstiidt," in discussing the antigoritic serpentines of 

 northern Sweden, says that tlie foliaeeous mineral always extinguishes light 

 when the cleavage plane coincides with a principal plane of the nicols, but 

 that when the cleavage plan is liorizontal the mineral remains dark between 

 crossed nicols. It does not dichroise sensibly. The interference colors are 

 often quite vivid, especially in the coarser, foliaeeous varieties, but are fre- 

 quently feeble and then change from black to grayish blue. 



' Phys. "iler Mineral., p. 372. 



-MiD.iiiic., p. 441. 



'Tscherniaks mineral. Mittbeil., vol. :!, 187.">, p. 183. 



■•Ibid., vol. 1, 18T8, p. 459. 



= Ibid., vol. !:i, 1883, p. Gl. 



^Geol. Foreuingeus .Stockbolui Forlia;icll., vol. 7, 1SS4, p. 358. 



