PLATE VIII. 



Fig. 1. Peridotite, — Lherzolite. Baste, Harz. 



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The first five figures of this plate form a series in the order of their numliers, showing progressive 

 alteration in the same type of rock. Fig. 1 shows a yellowish enstatite mass holding colorless 

 fissured grains of olivine, some of which contain brown picotite grains. The olivine cracks 

 sometimes extend into the adjace-nt enstatite, and even across this into the contiguous olivine. 

 The greenish color of the enstatite near the upper right-hand olivine grain marks an altera- 

 tion state. The color of the enstatite probably marks the beginning of a change 133,134 



Fig. 2. Peridotite, — Lherzolite. Baste, Harz. 



This .shows a further change in the same rock as Fig. 1. The color of the pyi-oxene minerals is 

 deeper and muuh iron-ore dust has appeared, as one of the first products of alteration, along 

 the fissures of the olivine. Bands of black ore-dust and yellowish serpentine cross the lower 

 portion of the figure. Yellowish and greenish serpentine replaces portions of the silicates. . 133, 134 



Fig. 3. Peridotite, — Lherzolite. C'hristiania, Norway. 



A brownish altered enstatite and diallage mass, holding greeni.sh serpentine pseudomorphs after 

 olivine. The structure produced by the formation of the serpentine along the olivine fissures 

 is distinctly shown, while colorless interstitial fragments of oli\'ine remain in portions of the 

 pseudomorphs. The bluish band on the right of the figure marks a border of alteration 

 between the original olivine and enstatite. Much black iron-ore dust is to be seen, par- 

 ticularly in the altered olivine, but it is not so abundant as it is in Fig. 2 134, 135 



Fig. 4. Peridotite, — Lherzolite, — Serpentine. G.i^rud, Norway. 



The pyroxene minerals are eluanged, having a brownish color, while in part they are replaced by 

 a grayish-white to colorless magnesium-carbonate, as shown on the left of the figure. The 

 olivine is altered more than it was in the preceding figure, only a very few granules remaining 

 unchanged in the yellow serpentine. The iron-ore dust has diminished in amount, but 

 suflicient remains to mark the original fissures in the olivine. The lithographer in his en- 

 deavor to be exact, has reproduced on the left and towards the bottom of the figui'e three 

 bubbles which wei'e in the balsam, but which, it is needless to say, were not in the original 

 drawing 135 



Fig. 5. Peridotite, — Lherzolite, — Serpentine. Baste, Harz. 



This shows a brownish altered pyroxene mass containing yellow serpentine pseudomorphs after 

 olivine. The change has progressed further here than in the preceding. The serpentine 

 has a more uniform and paler color, the iron ore diminishes in amount, and the oli^ane is 

 entirely changed 133, 134 



Fig. 6. Peridotite, — Picrite. Herbohn, Nassau. 



On the right of the figure is a browniish augite crystal containing olivine grains, some of which 

 is altered to greenish, bluish, and brownish serpentine and biotite. Part of the olivines show 

 the commencement of alteration only by the production of iron-ore dust and colorless serpen- 

 tine along the fissures. At the base of the augite is shown a greenish alteration product, 

 while a large olivine is attached to the upper portion. The groundmass is a yellowish, gray- 

 ish, bluish, and greenish serpentine, holding partially altered olivines, brown biotite scales, 

 and black iron ores ' 150 



