150 



PERIDOTITE. 



often associated with tlie augite. The quartz is in water-clear, beautifully polarizing fis- 

 sured grains containing Huid and glass inclusions, as well as apatite needles.* 



Inchcolm Island, Scotland. 



An apparent intrusive mass is described by Dr. A. Geikie as composed of a serpenti- 

 iious base holding honey-yellow grains of olivine, dark lustrous augites, and a few plates 

 of brown biotite. In the section the olivine is in a great measure undecomposed, though 

 presenting the usual exterior band and transverse threads of serpentine. The augite is 

 of a pale yellow color, and in large and well-defined prisms, often enclosing olivine. A 

 little milky plagioclase full of fissures and decomposition products was observed. Long 

 scales of rich brown biotite occur here and tliere ; also a few plates and grains of probable 

 titauiferous iron. One of the most conspicuous constituents is a rich emerald-green to 

 grass-green decomposition product, filling up the interstices and running in veins and 

 irregular streaks or tufts through the rock. Other pale or colorless aggregates, which are 

 sometimes distinctly fibrous, also occur. These various decomposition products some- 

 times show the polarization of serpentine, and sometimes that of chlorite. Some zeolitic 

 fibrous tufts were seen.f 



Herhorn, Nassau. 



The section purchased from Eichard Fuess is composed of pale brownish-yellow augite 

 and clear fissured olivine surrounded and held by the secondary serpentinous products. 

 The augite in places is changed to a pleochroic green fibrous mineral, wliose extinction, 

 being parallel to the nicol diagonal, presents a strong contrast in polarized light to the mono- 

 clinic augite. The augite holds numerous rounded grains of olivine, the same as enstatite 

 commonly does in other rocks. The olivine is in rounded fissured forms, surrounded and 

 traversed by the plexus of alteration material. Part of the latter is dichroic, varying from 

 a green to brownish-yellow, and from its relations to the secondary brownish-yellow biotite 

 it is regarded as a transition stage in the formation of the biotite. It is in irregular fibrous 

 forms, the fibres being crumpled and aggregated together. In other portions some whitish 

 fibrous material occurs, which affects polarized light in the same manner as part of the ac- 

 tinolite does in the cumhcrlandite. However, the chief portion of the alteration material 

 is serpentine. The olivine is in part very clear, and in part changed to a pale-yellowish 

 serpcmtine filled with globulites and margarites of iron ore. They are seen in the serpen- 

 tine veins ramifying through the olivine, and projecting like pseudopodia from the sur- 

 rounding material into the partially or entirely altered olivine. The ore also forms 

 black giains and crystals (some of which are octahedrons) in the olivine, and in the 

 network of alteration material. It frequently forms black bands, rows of grains, or fine 

 powder, along the fissures or centres and sides of tlie serpentine veins. The biotite 

 is in irregular yellowish-brown scales and grains, some of which are surrounded by the 

 black ore grains. It is strongly dichroic, and shows oftentimes a wavy, fibrous polar- 

 ization. (Plate VIII. figure 6.) 



Ellgoih, Austria. 



This rock is described by Dr. H. v. Mold as having a black to blackish-green serpen- 

 tinous groundmass holding many mica and hornblende particles. 



The section is in part a very fine tufted serpentine, and in part a scaly fibrous one, of 



* E. Hussak, Vcrliandl. Gcol. Reichs., ISSl, jip. 2.V8-2(52. 

 t Tiaus. Roy. Soc. Edin., 1879, xxi.x. 506-5OS. 



