PLATE IV. 



Fig. 1. Peridotite, — Lherzolite. New Concord, Guernsey Co., Ohio. ' 



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 This shows a grayish, crystalline, granular mass of olivine, enstatite, and diallage, containing 

 dark grains of iron and pyrrhotite. The groundmass is stained by the oxidation of the 

 iron to a reddish- and yellowish-brow n 95,96 



Fig. 2. Peridotite, — Dunite. Franklin, North Carolina. 



This figure represents a granular mass of olivine traversed by fissures, giving it a grayish appear- 

 ance. A little below the centre, and also on the left of the figure, are two dark chromite 

 grains. This, with the ten preceding figures, presents the structure of the unaltered peridotites. 118 



Fig. 3. Peridotite, — Dunite. Webster, North Carolina. 



This represents ar. early stage in the alteration of the peridotites, in which a greenish and 

 yellowish fibrous serpentine has been formed along the fissures of the olivine, leaving color- 

 less grains in the interstices of the serpentine network. A further stage in the alteration is 

 the change of some of the interstitial grains to a pale yellow serpentine. The reddish-brown 

 grains sprinkled with black granules show the picotite, while the minute black grains in and 

 about the serpentine indicate the magnetite produced during the process of the conversion of 

 the olivine into serpentine 119, 120 



Fig. 4. Peridotite, — Saxonite. Andestad See, Aure, Norway. 



This figures a further change in a peridotite, in which the chief portion is altered to a greenish 

 serpentine containing clear grains of unaltered olivine. In the upper portion of the figure 

 is a partially altered enstatite traversed by fissures and containing much secondary magnetite 

 dust. This enstatite is also partially altered to serpentine. Dark magnetite dust is shown 

 in connection with the olivine grains, while dark grains of chromite are figured in the 

 serpentine 126, 127 



Fig. 5. Peridotite, — Serpentine. High Bridge, New Jersey. 



This represents an extreme stage in the process of the alteration of a peridotite, in which is 

 shown a yellowish and grayish serpentine mass blotched with aggregations of dark iron-ore 

 grains. Extending across the figure is an irregular pronged grayish band, formed by serpen- 

 tinized olivine traversed by numerous fissures filled with dust-like granules of iron ore. 

 Enclosed in this gray band are greenish spots of partially altered olivine grains 157 



Fig. 6 Peridotite, — Lherzolite. Jaina Eiver, San Domingo. 



This shows a grayish-white serpentine mass holding dark patches of iron ore The yellowish- 

 brown mass to the right of the centre is a diallage crystal altered to serpentine, the four white 

 sjjots indicating the unchanged portions. The other yellowish-brown and greenish spots are 

 serpentinized pyroxenes and olivines, while the gray irregular band on the left is a partially 

 altered mass of diallage and feldspar 140 



