

PLATE VII. 



Fig. 1. Enstatite. Colusa Co., California. 



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This is fii^urecl from the same rock as Figs. 2 ami 3, Plate V. It indicates the manner in which 

 enstatite is altered to serpentine. The longitudinal cleavage runs from side to side, and is 

 crossed by the vertical fractures. The unchanged enstatite is colorless, but upon the borders 

 of the cleavage planes and cross fractures the mineral has l)een altered to a greenish and 

 yellowish serpentine. A yellowish serpentine vein runs from the upper left-hand portion of 

 the figure to the centre of the base 131,132 



Fig. 2. Peridotite, — Serpentine. Westfield, Masschusetts. 



This continues the series formed by Figs. 1, 2, and 4 of Plate V., and Figs 2, 4, and 5 of Plate VI. 

 Here the serpentine is of a pale yellow without trace of the usual network, and much of the 



black iron ore has been arranged in the form of a rectangular grating 159, 160 



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Fig. 3. Peridotite, — Lherzolite. Presque Isle, Michigan. 



This shows a grayish and greenish partially serpentinized enstatite containing black iron ore and 

 holding rounded olivines. These are fissured and rendered opaque in portions owing to the 

 precipitations of magnetite dust along the borders of the fissures. Greenish and yellowish 

 serpentine is also to be observed in connection with both the enstatite and olivine 136 



Fig. 4. Peridotite, — Lherzolite, — Serpentine. Presque Isle, Michigan. 



This is drawn from a portion of the same continuous rock-mass as Fig. 3, and represents a more 

 highly altered state of the rock. The enstatite and diallage are largely replaced by greenish 

 serpentine, bluish-green and yellowish biotite (?), gray dolomite, and black iron ores. The 

 olivines are in part still more opaque from the rejected iron ore, and they have so far been 

 changed to serpentine that comparatively few clear, unaltered, interstitial iragments remain. 

 The structure is confused, and the distinctness of the minerals confused by the alteration. . 136, 137 



Fig. 5. Peridotite, — Lherzolite, — Dolomite. Presque Isle, Michigan. 



This is from the same continuous mass of rock as Figs. 3 and 4, and displays a further stage in 

 the alteration. The groundmass is formed by a grayish mass of secondary granular dolomite, 

 which holds yellowish, bluish-green, and brown pseudomorphs of serpentine and ferruginous 

 material after olivine 137 



Fig. 6. Peridotite, — Serpentine. Fitztown, Berks Co., Pennsylvania. 



This shows a yellow serpentine mass containing grayish and colorless grains of olivine. The 

 brown masses represent secondary dolomite grains formed in the serpentine, but the lithog- 

 rapher has given them much too dark a color, since the grains figured are of a cloudy-gray 

 to brownish-gray color 152 



