152 PERIDOTITK 



Beside the magnetite, tliere occurs a whitish opaque mineral substance, which is con- 

 sidered to be magnesite. 



A similar rock, which comes from the nickel mine Hillfc Goffcs in the Weyherhecke, 

 Nassau, is described as a granular to compact serpentine of a dark-green color. In the 

 dark groundmass lie many minerals; as, for instance, calcite, magnesite, chrysotile, schiller- 

 spar, pyrite, clialcopyrite, and millerite. The olivine is much clianged to serpentine, as 

 also is the hypersthene. The chief difference between this and the preceding appears 

 to be mainly in the greater amount of alteration.* 



Variety. — Serpentine. 

 Mtztown, Berks Co., Pennsylvania. 



1562. Section : a greenish-yellow serpentine holding crystals of grayish-white dolo- 

 mite. Under the microscope in part of the section the serpentine is seen to form a net- 

 work following the fissures in the clear, unaltered olivine, and enclosing grains of it, the 

 fibres being generally parallel to the direction of the serpentine veins. In other portions 

 only traces of the olivine remain, while in still others only the structure of the serpentine 

 shows its origin. The direct conversion of olivine into serpentine is well illustrated in 

 this section. The dolomite is in rhombohedrons, and irregular, sometimes geniculated 

 grains. The larger forms mostly contain an irregular central portion of a clear, fissured 

 mineral, closely resembling olivine in its clearness ; but it is isotropic, and belongs to 

 spinel. One grain, hcrwever, showed a pale grayish color in polarized light. The dolo- 

 mite is surrounded by a wliite border of aggregately polarizing fibrous serpentine, which 

 in places occupies considerable of the section. The serpentine about the olivine is in 

 the middle of the fissures of a clear greenish-yellow color, but next the olivine frequently 

 passes into a clear nearly white serpentine. (Plate VII. figure 6.) 



Tlie rock is composed of a mi.xture of greenish and yellowish serpentine and grayish- 

 white dolomite. Many colorless and pale bluish octahedrons of sjiinel were observed in 

 the dolomite. Three of the sides of the specimen show a " .slickensided " surface coated 

 with serpentine, dolomite, and white mica (plilogopite ?)• This rock is an " ophicalcite." 



FranJcenstein, Silesia. 



Prof. T. Liebisch describes this rock as a siskin-green to oil-green serpentine mass 

 holding chromite. In the section it shows the usual network of serpentine enclosing 

 colorless olivine grains, minute crystals of actinolite, and whitish talc-like plates.f 



Lelc'd, Nonmy. 



The section shows portions in which the olivine is still unaltered, but it is rendered 

 cloudy by a magnetite dust scattered through it. 



In other portions the magnetite is very abundant, and in polarized light the rock is 

 seen to contain numerous plates and fibres, having a similarity to sericite, or to talc and 

 chlorite. It contains grains of a magnesium carbonate \ 



* Konrad Oebljoke, Iiiaug. Diss., Wui'zbur?, 1S77, 38 pp. 



t Zeit. Deut, geol. GeselL, 1877, xxix. 732. % Mohl, Nyt Mag., 1877, xxiii. 121. 



