172 PERIDOTITE. 



daiy dolomite that evidently have replaced a portion of the groundmass, and 

 in other parts of the section have j^i'oduced an ophicalcite. 



An interesting series of Iherzolites is shown in the first five figures of 

 Plate VIII. In figure 1 is represented a groundmass of enstatite enclosing 

 fissured and unaltered olivine grains holding picotite. The pyroxene mineral 

 is but slightly changed, showing this in one gi'eenish band — extending a 

 little distance from one of the upper olivines — also in the yellowish-brown 

 tinge of the whole mass. We next pass into a form (fig. 2) in Avhich the 

 pyroxene minerals are more changed and of a deeper brownish color, while 

 the boundaries between them and the olivine are less distinct. Again, the 

 separation of the iron ore-dust along the fissures and borders of the olivines 

 becomes strongly marked, and a series of black ore-bands extends across the 

 lower portion of the section. In figure 3 the alteration is seen to be still 

 greater, the color of the pyroxenes higher, and their cleavage j)lanes nearly 

 obliterated ; while in some cases a bluish border extends between them and 

 the olivine grains. The olivines are much altered to a greenish serpentine, 

 showing its netw^ork formation along the fissures, and with the iron ores 

 holding in the interstices some still unaltered portions of olivine. However, 

 in some of the gi'ains the whole mass has been replaced by serpentine. 

 Here, as elsewhere, it can be seen that while iron ores are the first pro- 

 ducts formed during the progress of the chemical changes which lead to 

 the production of serpentine, these ores either disappear during the sub- 

 sequent changes, or else are aggregated together in collections of greater 

 or less size. 



Figure 4 shows a still further progress in the alteration, the pyi-oxenes 

 being greatly changed and in some parts replaced by a grayish mineral, as 

 shown on the left of the centre. Only a very few portions of the olivine 

 have escaped the general alteration to a yellowish serpentine, while the 

 original fissures of the olivine are shown by the arrangement of the 

 remnants of the ore bands. In figure 5 the alteration of the rock has 

 progressed still further, the pyroxenes being entirely changed or nearly so ; 

 and the olivine completely altered to a pale yellowish serpentine, in which 

 only portions of the ore-bands ai'e still visible. 



This series of sections jiossesses an additional interest from its bearings 

 on the cozo'dii question, for they were examined by Dr. Wm. B. Carpenter, in 

 the presence of Mr. Alexander Agassiz and myself, regarding the occurrence 

 of eosoon in them. They belong to the variety of peridotite commonly 



