THE TERRESTEIAL PEEIDOTITES. — LHERZOLITE. 141 



Tudbnoos, Baden. 



5000. The specimen from this locality in the collection is a blackish-green compact 

 one, containing a few enstatite and diallage crystals. It weathers to an earthy rusty- 

 brown, showing the network method of decomposition frequently observed in the perido- 

 tites. This with the sections was purchased from Voigt and Hochgesang, Gottingen. 



One section has a greenish groundmass holding crystals and grains of enstatite, dial- 

 lage, olivine, and picotite. The enstatite is in part clear and unaltered, holding picotite 

 grains, and in part it has suffered a greenish and yellowish serpentinous alteration. The 

 same can be stated of the diallage. Both are in rounded crystals and irregular masses, 

 and show the usual cleavage lines. 



The olivine when unchanged is in clear grains, the remnants of the original larger 

 crystals and grains. That a series of these grains now separated by the serpentine bands 

 once formed the same crystalline mass, is shown conclusively by their possessing the same 

 optical orientation. The major portion of the original olivine has been changed to ser- 

 pentine, the structure showing tlie successive stages of alteration. The serpentine formed 

 first along the fissures has a dividing line indicating the fi.ssure, and on both sides the ser- 

 pentine fibres stand at right angles to that line. Tlie color of this serpentine is generally 

 a light yellowish-green. The interior portion occupying the interspaces left between tlie 

 network lines above described, is occupied by serpentine of a different shade of green, 

 sometimes lighter, sometimes darker. This serpentine, which replaces the oliviue grains 

 before described, shows not only by its color, but also by its structure, both in common 

 and in polarized light, that it possesses a distinct organization from that of the network, 

 and is distinctively a later product. The serpentine forms the chief portion of the 

 groundmass and is feebly dichroic. While it is usually of some shade of yellowish-green 

 to pale-green, in some cases, especially about the ferruginous products, it is of a bluish- 

 green color, doubtless owing to the ferrous oxide. Some secondary actinolite and talc 

 exist associated with the pyroxene minerals. The picotite is in coffee-brown and pale- 

 ereenish irregular "rains scattered throughout the section in the different minerals. The 

 larger grains along their fissures and edges are altered to a black ferruginous product, 

 probably chromite. This alteration sometimes extends nearly, and sometimes quite, 

 through the eutire picotite grain. Considerable secondary iron ore exists, which is either 

 chromite or magnetite. 



Anotlier section has a yellowish-green groundmass containing grains of enstatite, 

 diallage, and picotite, and traversed by veins of talc. The groundmass is a network of 

 serpentine of a pale yellowish-green color, surrounding portions of a deeper green repre- 

 senting the unfissured parts of the olivine, while the meshes follow the fissures. The 

 enstatite and diallage are in places only slightly altered ; but for the most part they are 

 traversed by threads of the serpentine M'eb, and possess a fibrous alteration-structure 

 showing a more or less aggregate polarization ; yet in the majority of cases they retain 

 their relative extinction. This serpentine is very beautiful in polarized light. The pico- 

 tite is in irregular fissured grains, sometimes opaque, but more commonly with dark 

 brown to black edges, and with a light brown to dark reddish-brown interior. Much 

 ferruginous material in grains and irregular patches is distributed through the section. 

 This serpentine has been referred by Eosenbusch to the Iherzolites. 



