62 KOCK-METAMOKPHISM. 



magnesia. Fayalite is usually classed with peridote, though 

 containing only a small quantity of magnesia. 



Serpentine is another chemically related mineral, with the dif- 

 ference principally in being hydrous. It frequently contains iron 

 protoxide, by which its relation to peridote is further sustained. 



The frequent change of peridote into serpentine, which we 

 have already noticed, has no doubt largely contributed to the 

 prevailing idea that the former mineral is always an original 

 product. Taking this view, some difficulty would be felt by 

 conceiving that peridote itself could occur as a product of pseu- 

 domorphism. Hence it is, in the case of a rock containing 

 peridote associated with augite or hornblende, that the idea of 

 the first having been pseudomorphically generated out of either 

 of the latter two does not seem to have been entertained. 



Rocks containing peridote have been called peridolytes, the 

 principal of which are Iherzolyte, dunyte, and picryte. 



Peridote is of somewhat common occurrence in many dole- 

 rites, trachytes, and lavas. Its presence is well known in what 

 may be an igneous rock, the hypersthenyte near Elfdalen in 

 Sweden*; and it is said to occur in the granite or syenite 

 between the Nile and the Red Sea. Delesse discovered a ferru- 

 ginous variety of peridote in cavities of the pegmatite or granite 

 of the Mourne Mountains f. The related species, fayalite, has 

 been found in ordinary metamorphics (those simply mineralized) 

 at Tunaberg in Sweden, forming, with augite &c., a bed, called 

 eulysyte, in gneiss. A true peridote dominates in a rock, called 

 olivenyte, associated with talc-schist, at Uddevalla, Sweden ; a 

 variety has been observed near Kyschtimsk, north of Miask, 

 and near Synersk in the Ural in talcose rock : these cases 

 show that the mineral is also present in methylosed metamor- 

 phics ; while its frequent occurrence in serpentine, made known 

 by Breithaupt, Bischof, Hunt, Otto Hahn, and others, leads to 

 the same conclusion. As it is found in methylosed rocks, the 

 presence of peridote in veins of calcite, presumably of secondary 

 origin, traversing large blocks of talcose schist scattered over 

 the southern moraine of the Findelen glacier near Zermatt, is 

 not surprising to us, though it must be to those who subscribe 



* This rock is said to consist of hypersthene, labradorite, and peridote. 



t Reference to the fact is unknown to us ; but the occurrence of peri- 

 dote in the Mourne granite is mentioned by Dr. Haughton in Quart. Journ. 

 Geol. Soc. vol. xii. p. 191. 



