ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC. 63 



to the prevailing idea just stated. But it ought to be greatly 

 embarrassing to the latter to find that it notably occurs (as bol- 

 tonite) in the hemithrene of Massachusetts, also as fe roundish 

 sharply defined grains/' more or less changed into serpentine, 

 crowding the corresponding rock (" granular limestone >} ) at 

 Snarum, Aker, and Modum in Norway*. 



To add to its remarkable indifference as to the nature of the 

 material selected by peridote for its matrix, the fact may be 

 mentioned that this mineral, or some of its varieties, besides 

 being often in association with other substances, exists imbedded 

 in the nickeliferous iron of meteoric amygdaloids that have fallen 

 in Siberia, Atacama, and elsewheref ; and it is a common ingre- 

 dient in meteoric stones. 



We have shown that peridote is a variable mineral through 

 the increase or decrease of the percentage of its magnesia and 

 iron, also through the presence of accessory substances. Thus, 

 let it be conceived, in the case of a rock like ophite, which has 

 often resulted from hornblende and augite, that during the 

 process of alteration the protoxide of iron belonging to either 

 of these minerals were not removed, there seems to be no valid 

 reason against the probability that this substance would become 

 united, all things being favourable, with any available free sili- 

 cate of magnesia, and thus form peridote or some variety of it. 



The two minerals peridote and serpentine can be readily 

 differentiated by the polariscope when they are associated as in 

 ophite. Guided by the peridote in the hypersthenyte of Elfdalen 

 (which is optically in agreement with the same mineral from 

 Expailly, in Auvergne, and several other localities), and allowing 

 for variations due to circumstances such as have been mentioned, 

 it may be stated that this mineral exhibits a display of the 

 richest colours by rotating the prisms, viz. yellowish green, sap- 

 green, pink -rose, ruby, blue, &c. being a greater variety and of 

 more brilliancy than those displayed by chrysotile. On the 

 contrary, serpentine only shows pale yellow (under parallel 

 prisms), changing into dark grey (when the prisms are crossed), 

 and back again into pale yellow. 



The specimens of peridote which we are about to notice exhibit 



* Neues Jahrbuch fur Mineral ogie, 1870, p. 828. 



f As meteoric peridote is generally believed to be of extramundane origin, 

 probably solar, obviously any endeavour to explain its origin may be properly 

 avoided. 



