ITS MlCROSCOl'IC CHAFlACTKItS. 170 



of the section is composed of serpentine, olivine, biotite plates, iron 

 ores, etc. 



In the course of the conversion of peridotites into serpentine, it has heen 

 seen that the original characters of the rock were gradually obliterated, 

 giving a texture to the serpentine that showed its mode of formation. IJut 

 it was found that on further change, patches appeared which showed no trace 

 of the mode of formation, and that these gradually occupied the entire rock- 

 mass, yielding specimens whose microscopic characters gave no clue to their 

 origin. In these extreme alterations a more or less schistose structure is 

 often produced in the rock, and from this, the writer conceives, has arisen 

 the view already referred to, that serpentine rocks are derived from 

 schists as well as from massive rocks {ante, pp. 144-147). The absence of 

 the reticulated or mesh structure in the serjDentine, and the supposed want 

 of chromite and picotite appear to be entirely due to the great alteration 

 which produces a structure in the rock almost if not quite identical with tlir.t 

 of serpentine veinstones. The only apparent difference between the mode of 

 formation of these serpentines that are homogeneous and the serpentine 

 veinstones, appears to be that in one the molecular or chemical changes have 

 taken place in the body of the rock, while in the other a transference to a 

 new locality has been superadded. There may be mentioned amongst the 

 various products of alteration found in the peridotites — besides serpentine, 

 dolomite, and iron ores — the following : actinolite, hornblende, smaragdite, 

 quartz, zircon, spinel, garnet, feldspar, talc, chlorite, biotite, and various 

 other micaceous minerals and carbonates. 



For the special variations and further details, the reader is referred to the 

 descriptions of the specimens from different localities. The points that 

 seemed to the waiter most important, in the case of nearly every peri- 

 dotite or group of peridotites which has been microscopically studied, have 

 been presented in the preceding pages. This was necessitated by the small 

 number of Cordilleras peridotites at hand, and our still imperfect knowl- 

 edge of the group, which seemed to demand somewhere a tolerably complete 

 summary of the observed cases and their combination into a connected 

 series. 



