THE TERRESTRIAL PERIDOTITES. — PORODITE. 161 



Nev'port, Vermont. 



95. This rock -n-as collected by Mr. J. H. Huntington, near ]\Ir. C. Gilpin's residence. 

 It is a grayish-green rock having a greenish serpentine groundmass, holding grayish and 

 brownish masses and crystals of a ferruginous carbonate, showing well-marked rhombohe- 

 dral cleavage. 



Section : a gray mass traversed and sprinkled with black iron ore. Composed of a 

 coarsely fibrous serpentine, a dirty gray, feebly polarizing carbonate showing well-marked 

 rhombohedral cleavage, and iron ore. The serpentine forms the principal portion of the 

 section. 



Cdinac, Austria. 



A light-green rock containing dark particles of amorphous silica. The section shows 

 the usual network of serpentine witli isotropic silica and picotite.* 



Texas, Pennsylvania. 



A clear leek-green serpentine, obtained from W. J. Knowlton, 169 Tremont Street, 

 Boston, containing chromic iron in veins and crystals, or scattered granules. 



Section : clear and grayish-white, with black spots of chromic iron. The clear portions 

 show tlie fibrous polarization of serpentine, some of the fibres being distinctly optically 

 orthorhombic in character. In the grayish portion lie plates and fibres of tremolite ; 

 M-hile the chromite is opaque in transmitted Light, except in the thinnest portions, which 

 are translucent and reddish-brown. By reflected light the grains are dull, and without 

 lustre. The larger grains are traversed by fissures filled with serpentine. A grain of 

 pyrite was observed. 



Cliester, Pennsylvania. 



1487. A clear leek-green compact rock containing grains of chromite. Weathers 

 yellowish and greenish-gray. Section: clear grayish-white fibrous mass showing the 

 usual aggregate fibrous polarization. 



This and the preceding one show the extreme change in rocks to pure serpentine. 



YAr.iETY. — Porodite. 



Fatu LnJca, Timor. 



Large, rounded pieces of serpentine cemented by pale-whitish and finely spherulitic 

 opal and a greenish paste. The serpentine is yellowish and grass-green, and shows tlie 

 mesh structure. The history of these altered fragments is summed up by Wichmann 

 as follows: — 1. The formation of the peridotite. 2. The formation of the serpen- 

 tine, including the separation of the iron ore and the beginning of the Masclicnstrudur. 

 3. The decomposition of the iron ore and the formation of the yellow network 4. The 

 impregnation of the interstitial portion with the alteration-product, — chromite. 5. The 

 alteration of the exterior into a finely fibrous compact gray substance. 



The cement is mostly serpentine with a fine scaly material. f 



* C V. JoLu, Jahr. Geol. Reicbs., 18S0, xxx. 41S. f Jaarb. Mijiiw. N. 0. I., 1SS2, pp. 216-222. 



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